In single-voice staves, all rests should be left-aligned. It's equivalent to the position if it were a note head. The only exception is the center-aligned whole rest.
When doing multiple parts on a staff, the rest is centered between the left-most and right-most edge the notehead. For example, a quarter note major 2nd in one voice on a single staff and a quarter rest in another voice on that same staff.
I don't think it's that significant. I've seen it done differently by different engravers.
These two books are great:
Especially the first one, which includes just about every possible exception. It then goes into unconventional 20th century notation practices like proportional notation and graph notations.
 I highly, highly recommend that first book It's very comprehensive. It's also fun to just look at because the book predates computer engraving and was made at the time notation was absolutely perfect.