We've had many threads on the leveling of a rifle and aligning the scope, enough to know that its not always as simple as its been stated. Turrets aren't always perfectly square to the reticle, rails aren't always level on the action, etc., etc., etc. Which was what prompted me to ask. The levels that mount on the rail assume that the rail is level and square on the action, but is it? Perhaps if the rail is machined into the action, but what about those that are separate parts? The levels that mount to the scope tube are nice in that you can adjust them so that they are level relative to something.
I'm pointing this out because when you're at the point where you need a level then what it is level to needs to be considered. It is part of a system, not a stand-alone device. Merely attaching a level to a rifle doesn't mean that the rest of the system is square and true unless all of that has already been set-up or verified to be square and true.