I've built 3 wheels now without a spoke tension meter. I just pluck them and listen to the pitch. As far as what's the correct tension, a friend of mine said "make them as tight as you can until you think they're about ready to strip." On the three wheels I've done this for, I've not had one of them ever go even slightly out of true after > 10,000 miles including fairly rough road that destroyed factory wheels.

Sometimes a factory built wheel is just garbage. Sometimes not. A good, hand built wheel should last for many years of normal use without ever seeing a spoke wrench. I bought a set of wheels a few years back and had all kinds of problems with the spoke tension. I ended up taking all the spokes out, buying a new set and then building it up by hand myself. That wheel lasted me several years of hard mountain biking and never needed to be trued after the initial build. If there hasn't been any real trauma to the wheel then just have the spokes relaced by hand and all should be fine.

A properly built wheel should require no "regular maintenance". Only if there is physical damage should one need to do anything at all.

When you do your job right they stay trued and tensioned until you bend the rim on a road obstacle unless you wear out the braking surfaces before that and need to rebuild the wheel while it's still otherwise in fine shape.

I got 12-14 years out of my last front wheel without touching it until breaking a leg, getting fat, and learning that 200+ pounds of load on a 400 gram rim doesn't last.

Wore out my big ring, got a spontaneous down-shift, crashed, and only got five years out of the last rear before I needed to deal with it.

(emphasis mine)

I'd be careful about the equipment combos you're using when employing this method of spoke tensioning. I used to build based on pitch, and it's a fine method; never had a problem, as long as you've got a reference wheel to base it against... Same hub, same rim, same spokes being used.
Depending on the wheel components, you could over-tension a wheel by cranking everything down until you think it's about to snap. Depending on the degree of over-tensioning it can cause a number of issues, some not immediately apparent. If it's severely over-tensioned, then just inflating the tire on the rim can put the whole thing out of true. If it's mildly over-tensioned (per manufacturer's specs for the rim or the hub) then you can end up with spokes pulling through the rim, eyelets ripping out, or cracked/broken hub flanges.

When I build, I like to use my Park TM-1 to check the tension multiple times around as I'm bringing the wheel up to final tension. I find that if I start with a true, round rim and use a nipple driver to start the spokes all evenly and then work an equal number of turns per spoke as I go around the wheel, I end up with a fairly even balanced tension and the wheel requires very little additional tweaking to bring it within +/- 0.01" of true.
Being a little bit larger than the average rider and putting some additional strain on wheels (CX, singlespeed/fixed) I run my spoke tension towards the upper end of the suggested range for whatever rim/hub combo I'm using. However, for the very same reason (singlespeed/fixed) I can also keep things under the ultra-high tensions that some Clydes prefer for a rear wheel since I don't have any drive side dishing to concern myself with.

All that said... *whew*

Were you (OP) breaking spokes at the heads or the threads (near the hub or near the rim)? Undertensioned spokes typically break at the heads (near the hub). Some things to consider are having a build done using spoke washers or using DT Alpine-III spokes (if they fit your hubs) which have a larger diameter at the head to reduce chatter/flex at the bend.

After I broke a spoke on my rear wheel, my LBS told me they replace a spoke once. If a second spoke breaks, then they rebuild/replace the wheel. A single spoke breaking could be a fluke. Multiple spokes breaking is a sign of bigger problems.

With my spoke replaced, I snapped a second one on my first ride. The good news is I had an excuse to upgrade my wheels to a nicer set.

My guess is 1% or so. I figure no more than 20% of bikes purchased at an LBS have the wheels built properly by machine or tensioned by the mechanic, and 95% are BSOs from XMart.

Or, if you're aware of the way things work, get the shop to tension it up before you roll the bike out the first time.

At the heads, all three of them.

I just noticed I had a broken spoke on my rear wheel this morning. I just started commuting with a pannier this week - could the extra weight have caused a non-perfectly adjusted spoke to have broken?

Sorry if this is a silly question, but I don't know much about spokes and wheels.

Yes, the tension problem becomes exaggerated the more weight you put on the bicycle.

To be clear, one spoke out of tension and the rest OK is unlikely. With some effort, you could true a wheel in that configuration. In that case, the low tension spoke would not be bearing a lot of load and wouldn't be that likely to fail. More likely is that many or most/all of the spokes are incorrectly tensioned (too low) if your wheel is somewhat true and spokes start to break spontaneously.

You mentioned your taint-butt was burning through pants, but now it's also corroding metal? Now there's a super power I hadn't seen.

x2

I'd have the wheel rebuilt with all new spokes and tell your builder to run the tension on the high side to account for extra weight from commuting. Basically, build it like a touring wheel.
While there's argument back and forth about strength differences using straight vs. butted spokes, I prefer to build all my wheels with straight gauge 2.0 (14g) spokes. The vibration absorption qualities that are typically alluded to when discussing the merits of butted spokes are all but gone when you move to a tire above 25mm width. The lower pressures and higher volumes of 28mm+ tires more than make up for the excessive stiffness of a straight 14g wheelbuild under higher tension.

great, now there is coffee on my keyboard.

and a new superpower emerges to go with the supercorrosion. He can make coffee appear in people's keyboards from a distance.

Counter-intuitively, bicycles stand on the lower spokes which see a decrease in tension with minimal changes elsewhere in the wheel.

Barring damage (like shifting the chain into them) spokes fail due to fatigue, with the number of cycles dependent on average stress (where the elbows in non-stress relieved spokes have areas with high average stress because they were never taken past their elastic limit) and magnitude of the variation (which is obviously higher for bigger riders and/or with luggage).

Well, there is a huge thread somewhere in the mechanic forum about that argument which i never got to the bottom of. The upshot seems that the effect is the same either way, excessive stress, fatigue, failure.