Even if you level the reticle, which is what I do, if it's off kilter w.r.t. the turrets then the inputs from the turrets won't be the natural orthogonal vectors implied by the coordinate axes, i.e. the reticle.
Consider an extreme example, a 45 degree can't. You want to move bullet impact one unit vertically. A one unit change on a turret will then produce .707 units of vertical and .707 units of horizontal change.
Love it when you talk dirty like that!
FWIW, I've always leveled the reticle too. A while back, Graf's threw a Segway tool in my order just for grins, and it really helps. I also place a little machinists' rule against the eyepiece, in line with the bars on the Segway, which gives me a reference line all the way across. That makes it easier to compare the reticle to the bars. Any difference at either end of the horozontal is easily seen.
