The important part of being level is that the scope is level to the world when you fire the rifle.
It makes no difference if the scope is level to the rifle or not. You can cant the rifle 45 degrees, but if the scope is level (horizontal stadia parallel to the actual horizon), the gun will shoot properly.
So what you want is a level that attaches to the scope to check for level when you are ready to fire.
Anyone here gone to the trouble of lapping their scope rings? Or is that getting a bit too anal for rifles that are not going to be used for competition shooting or taking a 2,000 yard shot?
If you get quality rings (not cheap Wally World) you will do more damage to the rings that you will fix.
Lapping removed the surface hardening (anodizing) of aluminum rings, causing them to corrode and wear.
When I did my .308 precision rifle recently, I asked this, and the scope dealer and several others convinced me to NOT lap the rings.