I'm not quite sure that there are any "rules of thumb" per se floating around out there, but I do know that some people can be very particular about every aspect of the cards they read with. Personally, I'm fairly laid back with regards to the backs of my cards (although I am spoiled right now, I read from the Gendron, and their backs are absolutely gorgeous).
From what I've seen out there, I know that some decks have a rougher or B&W depiction of whatever card they happen to be as their back. I've seen it in one deck in particular on Aeclectic, but I can't remember what it is right now. Personally, I'd hate to use a deck like that simply because I believe the cards should be judged individually as you turn them over and then again as a whole once the entire spread has been laid down. Picture backs let you cheat =P. I also prefer ambiguous backs that look the same upside down as they do right side up, but that's only because I read using reversed cards, and I don't like to know what is reversed and what isn't until I've got them laid out in a spread.
All that being said, its all about personal preference. Shop around and see what feels good for you! I hope this helps!