I guess I am asking for flames, but since I don't really care, here goes.
I am far from an expert marksman, but I took like 500 semesters of math and physics, and even though I have forgotten most of it, basic trig is not a big challenge for me. I have to say that I am not sure about the whole leveling business, according to my understanding, which could well be wrong.
Okay, so you're shooting a round that drops 10 feet at x yards, but your scope is canted away from the direction of gravity by one degree. So you think "down" is 10° from where it really is. Unless I'm missing something, that means your horizontal error, assuming everything else works perfectly, is 10 feet sin(1°). Before you tell me 10 feet isn't the hypotenuse, let me just say you should think twice.
So the horizontal deviation or error or whatever is about 2", always in the same direction. For 5°, it's about 10". I think a 5° cant would be pretty obvious to anyone who isn't tanked up, but it would be good to hear from someone who actually knows from experience, which isn't me.
Is any of this wrong yet? I can't see any obvious problems.
So for 6.5 Skinny Jeans at 1000, I get 5" at 1°, assuming a 285" drop. For 3°, I get about 15". So leveling must matter somewhat once you get far enough out for the round you're shooting. Is that right?
But assuming you get your scope level to your rifle to within a fraction of a degree, isn't it going to be a royal pain getting the center of the reticle and the axis of the bore in line, precisely, with the direction of gravity? I mean, if your scope and rifle are aligned, but your bipod is off, you're still off.
Also, it seems to me that aligning your scope with gravity is better than aligning it with the bore. It seems like the maximum error you could get from being canted with respect to the gun would be the horizontal distance from the bore's axis to the scope's axis, because it won't open up downrange. Is that wrong? But if your scope is canted with respect to gravity, you can be a lot farther off.
I've used machinists' parallels to set scopes level with rails, but don't ask me whether the rails were aligned with the receivers or whether the reticles were aligned with the bottom surfaces of the scopes, which I used as reference surfaces.
Seems to me that if you really want to do things as well as possible, you need to align your scope with your gun as well as you can, once, and align the scope with gravity every time you put your gun on a rest. Then shoot to see how far off things are in practice, and rotate your scope to match the error.