i'm not sure if it's saying that you shouldn't do it period or if it's just that you should be consistent
i might agree that the rule applies more the lower the resolution, since lines like that are a lot thicker and more noticeably different in a 16x16 sprite than a 64x64, however i don't think i've ever heard it strictly as a rule like that.
in fact i'd encourage the use of it, the way mario uses it in his bunny seems perfectly fine.