My understanding is that it is not essential that the riser and limbs are perfectly vertical when you shoot, as long as the bubble is centered (meaning the pins are perfectly vertical) you will hit behind the pin on a good shot at any distance. Therefor, part of setting up the bow for YOUR maximum accuracy and consistency, is rotating the sight so that the bubble is level with YOUR neutral grip on level surface (or whatever surface you want to be the default) to avoid torquing the bow to get the bubble level. Torque is very hard to reproducibly apply.
I don't recall the exact logic there, but the basic idea was that even at a 5 degree riser angle relative to vertical (enormous angle), the rest is only ~0.3" offset horizontally from the sight pins (which are usually ~4" above the rest). If you sighted in your bow at 60 yards, the arrow would hit about 1/10 of an inch left or right at 20 yards on a perfect shot. For most guys that's negligible relative to their average group size at 20 yards, and a lot better than forcing the bows riser to be perfectly vertical if its not naturally sitting there. Wouldn't want to sight in at 10 yards, unless you purposely sighted in for 0.33" left/right of where the pin was at that distance. Then you'd consistently hit 0.33" left/right of the pin at all distances, which would be fine with me for hunting.
Anybody have a link to others more knowledgable than me on this topic?
-Tim