How much wiggle room do you have for spoke length?
 

It would just be the amount of thread available for truing and the point where the spoke end on the nipple, right? How comfortable you are with running out of thread, not having the spoke go to the end of the nipple, or having the spoke go beyond the end of the nipple.

How much wiggle room is that though? Maybe 5mm?

Two. Possibly 3. Five milimeters is too much. It is also better to go too long than too short. Having spokes that are too short puts a lot of stress on the shoulders of the nipple and can result in nipple breakage or even spokes breaking at the threads.

My only addition to Stuarts good reply is that in a "U" shaped rim section (and not a "box", "double walled" or hollow section rim) is in play a too long spoke that extends past the top of the nipple will want grinding down flush with the nipple top end. Or suffer potential tube pokes.

Back in the day when "U" section wired on (clincher) rims were the norm, and bike companies were more dependent on the LBS to do the finishing work on their bikes, it was common for wheels to use spokes too long and these would stick up. Raleighs were well known for this. We had a 1/4" thick grinder wheel on one of the bench grinders with the guards removed. We used this spoke end grinding tool frequently, too often IMO. Andy

Nipples too long: On inexpensive single-wall rims, the spokes cannot protrude above the nipples, or the sharp ends will get through the rim strip and puncture the tube. On double-wall rims, I have built wheels in which the spoke ends are a mm above the nipples and not lost sleep about it.

Nipples too short: think of a cross-section of a spoke threaded into a nipple - if the spoke is sitting properly at close to the end of the nipple, then the nipple is in compression, and is very strong. If the spoke is too short, then the nipple is in tension, and is much weaker, particularly if alu.

+/- 2mm is the wiggle room I work with. Get your measurements as close as possible and stick within that 2mm, you can go over by .1 or .2 if need be but I would rather two separate spokes then too much of a deviation.

1mm.

I agree with this. 5mm is too long in my view.