I agree... they do even make a difference at 100 yards. It's amazing how the roll of a hill or other things in the background will make you believe you are level, when you are not at all.
I even noticed the first couple times I shot with a bubble level that I unconsciously kept trying to level my reticle to the target I haphazardly stapled up down range. And obviously that target was not even close to level.
Just keep in mind that even in higher end scopes, it's quite common for the reticle to not be 100% with the turrets. So if you dial for elevation, you'll want your bubble level matched mostly with your turrets. If you hold off for elevation, you'll want your bubble level matched mostly with your reticle.
I stay away from the bubble levels that mount to the picatinny rail or other places on the rifle. Get the kind that mount to the scope tube. WAY easier. If it's on the scope tube, then you don't need to be concerned with if the scope is also level with the rifle. Also if you have to move your scope around, then you just leave the bubble level set exactly how it is no matter what rifle you put it on. Hope that makes sense.