To rephrase what Jack is saying...
If an optic is oriented in relation to gravity such that the reticle and turret adjustments are square, and it is mostly centered above the barrel (and dead center would be nice, but practically speaking, not that significant), you're good. If the optic is mounted with a 45 degree offset, and you rotate it 45 degrees to use it, and it's still above the barrel (it should be pretty close, it should be fine. If you're not rotating it 45 degrees to shoot it, that's probably pretty uncomfortable.
From a geometry standpoint, the problems most people have in terms of scope level, isn't whether the scope itself is precisely leveled in relation to the 'level' reference point of the rifle, but the consistency of level during shooting itself, and that variation really shows up at distance, where gravity and the vertical angular offset built into the optic/barrel alignment to compensate for vertical drop then gets tilted away from fighting gravity, and simply driving lateral error, and impact will drift horizontally towards the tilt, and slightly lower, based on the degree and direction of the tilt.
As such, if you're sighting in at distance, you probably want to first level your target, and shoot something with grid lines, assuming your reticle allows that with horizontal/vertical lines, so you can eliminate variations of tilt during shooting as much as possible, unless you've got a level you can see on the inside of your scope. I have heard of such things, but never had one.