It is a sin.
You can tell if it is off balance by listening to it while it spins by listening to it (the sound would come in 'waves' with each rotation) or by spinning the wheel while the bike is on/in the repair stand. Spin the wheel vigorously: if the bike bobs up and down a bit from the motion of the wheel, the wheel has poor balance. A balanced wheel spun vigorously will not move the rest of the bike.
Does it matter? Some people are more particular about this than others. Supposedly, performance wise, it makes a difference. I can see the reasoning there.
As for wheels already being pre-balanced: not necessarily. There is no way to predict the length of the valve a person will use, so how would a mfg properly balance the wheel in anticipation? You, however, the user, tend to stick to the same length/type of valve... so you've then added in a constant weight, you can then balance from there. You can also be lucky by happening upon the right wheel + valve combo which results in a balanced wheel.
For the 'spreading out' of weight: it will be fine.
You could also, theoretically, take that same determined weight that you need to balance opposite the valve and apply it to the other 2 quadrants of the wheel, making an overall more
balanced wheel. But really, that's just heresy.
I just simply helped natefontaine with a better solution to what he wants to achieve. I have no strong opinion on whether a person should or needs to balance their wheels.
My current wheels do not need balancing, so I did not need to add weight.
|| Other projects in the works.