Originally Posted by FBinNY

The problem with MrRabbits case, is that even though the wheel wasn't dished properly and was somehow forced to an angle in the vertical dropouts, Aligning the hanger to that wheel, would have solved that bikes problems for that wheel, since alignment to wheel is what counts for RD performance.

You raise a niggling point, that though correct in theory, wouldn't be in practice. A wheel incorrectly dished bad enough, and canted in a frame to compensate (applies more to horizontal dropouts) will manifest other issues, such as sitting off to the side in the seat stays, or poor handling.

I do agree that having a reliable frame of reference is important, and the more likely issue is a wheel not pocketed fully in the dropouts -- which is common when wheels are installed with the bike in a repair stand -- can cause an error when used as the frame of reference. For 45+ years, I've made it a practice, and taught mechanics to do so, to ALWAYS mount wheels with the bike on the floor, letting gravity position the axle. This makes for a repeatable mount, uninfluenced by mechanic touch. Usually the wheel isn't actually installed on the ground, but the bike is taken from the stand, and both QRs opened and closed with the bike on the floor before brake and derailleur adjustments are made.

I often stop on the road to help disabled riders (mostly flats) and often find brakes rubbing when the wheel is replaced, because the wheel had originally been off, and the brake positioned accordingly.

But this is besides the point, because a wheel would need to be seriously off, for a dish issue to affect the hanger alignment enough to be an issue.