I find that different levels, more so the cheap ones, aren't consistent enough to trust their level.
Then sometimes the tops of the turrets aren't plum to the crosshairs because the reticle isn't plum to the turrets.
When I do level the horizontal crosshair it appears canted down on the left side which visually I hate. It's an optical allusion but it bothers me nevertheless.
Years ago I started putting the reticle oriented by how it looks correct for me when it appears plum and level.
I've won plenty of rifle matches with the scope set up how I like it on all kinds of rifles. That's both holding over and dialing.
I won a ELR match and hit the 2356 yard plate on the first try but missed the other two shots. The vertical capability of my reloads, that my dope was slightly off, and the many wind zones on out to the targets blowing my bullets, were the major problems. No I didn't use a level.
I'm not saying that my way is the right way because I'm quite sure my scopes aren't set up perfectly level and plum. I'm just saying that 99% of my misses are not related to how level the scope is, or isn't, "within reason" of course.
When shooting paper you'll usually have sighters bulls to practice on so how level the scope is(within reason) almost becomes irrelevant.
When shooting steel, like in PRS or NRL, you won't be steady enough when shooting off of obstacles to see a difference and most people don't have the time to make sure each shot is leveled.
Heck it's usually the wind that is the main problem that gets all of us.
And gun or gear problems, mainly POI shifts when it comes to airguns.
When I look through a scope I'm automatically pluming the reticle because I hate it when it looks canted. Yes I might be off a bit and I know the math doesn't add up on paper but when I'm shooting well I make the projectiles go where they should according to my abilities as well as within the realities of the imperfection I deal with.
Don't sweat it too much is what I'm getting at.