A few threads here have said how carbon can be damaged by overtightening various bolts particularly seat post clamp . Thus the need for a torque wrench with a carbon bicycle.
But what about steel or AL any issues there?

Got no sense of how 'Just tight enough' is by feel, and experience ? then get the torque wrench out for that and go by the Numbers.

With steel particularly there are two likely modes of failure: 1) not tight enough and the parts slip or 2) too tight and the bolt breaks..

Until you get a feel for what 5Nm feels like, a torque wrench or torque key is a good idea when dealing with stem bolts in particular. Aluminum can crack if tightened too much, but more likely failure mode is stripped out threads. If you really ham-fist a steel bolt going into threaded aluminum, the aluminum threading can strip out.

yes, you can damage anything by overtightening it. steel is the most resilient and can be bent back. the bigger issue is stripping aluminum bolts. aluminum frames on the other hand CAN be damaged and don't like being bent, but they are usually tough enough to get it snug by hand without overdoing it.

as bob said above, if you don't have enough experience to feel it or to know which things need only be snug and which things need to be torqued, then just get a torque wrench. it's a worthwhile investment for your home tool kit anyway.

I had to straighten my stem, it was a few degrees off center. Being that it's CF, I got out the torque wrench, set it for just under 6nm and got to work. Tightening the bolt I was surprised when the wrench clicked so soon. So surprised I loosened the bolts, reset the wrench and tightened it again. It was the correct torque, and with out the torque wrench I would have way over tightened it by feel. They aren't expensive, and could make your stuff last longer.

Bottom brackets and crank bolts are usually spec'ed for around 30 ft/lbs of torque. (Imagine a 30 pound weight hanging on a foot long wrench.)

Left to my own judgement I would probably under torque those parts. There are enough threads about left crank arms falling off and being ruined by under torqueing that I'm pretty sure it's a quite common issue.

Harbor Freight has inexpensive clicker torque wrenches, often as low as $10 on sale. You will need at least two to cover most bike tasks, 1/4" and 3/8" drive. My BB tool has a 1/2" drive recess. All of my HF wrenches were well within the 4% spec when tested on the torque analyzer at work.

If you buy a clicker be sure it clicks in both directions, not all do; think English threaded BBs.

I started a thread about torque wrenches, a beam style was recommended and I picked one up for 21.97 shipped on eBay. For the cost I figure why not have one, better safe than sorry but it was when I was putting together my CF bike.

Sometimes under-torqued crank arms get loose and ruin the aluminum where it mates to the crank spindle.

Under-torqued cassettes can be an issue too.

You can break bolts and strip threads by over-tightening, although hex keys and wrenches are scaled to fit the fastener so that's unlikely - "snug" once a fastener stops moving is usually correct.

Using a socket wrench over-torquing small fasteners is likely.

x2... with a note that many/most folk tend to overtighten. Even with experience, it takes discipline to not do that.

People tend to overtorque small fasteners and undertorque large ones.

Get a used Snap-On on eBay. My best suggestion is don't buy a cheap one. It may cost you more in the long-run.

Seems like many of the bicycle companies are demanding properly torqed bolts to qualify foe warrenty.

Agreed. Cheap often equals false economy, at least in my experience.