For prs matches or things of the sorts that involves multiple shots under time, then getting good at not having a level should probably be a priority.
If you're a brand new military guy just about to go to sniper school, probably expect to not use a level, and get use to using the reticle and the gun itself to guess how straight you are.
For everything else, a bubble doesn't hurt anything. Yes there are some inconsistencies in bubbles, namely in different temps but you'll have just as much inconsistency with lack of experience and shooting on the side of a hill at something else on the side of another hill. It's hard to tell what's truely vertical. FYI accuracy first and some other companies made a mercury bubble level to overcome such issues.
Generally speaking if you're fundamentals are good you don't need a level, but if you a shooting from various terrain, constantly without training into it, a bubble level is a way to help, like a muzzle break on a 6.5 creedmoor for those that can't seem to control their rifles recoil.