I try to take a lack of a back shot as a warning sign -- but a warning sign which way?  Some girls don't like their big butt and work out hard to get it small and tight.  They might not include a back shot to lure the customers who like small tight butts.  Other girls might know their butt is small and not as impressive as their rack, and so do not include a back shot to lure the customers who prefer bigger butts.  Instead of my finding out in person, my ability to pick only from those who meet my interset would be served better served by a simple rating of whether its flat, small, average, bigger, bubble, large, or huge.  

Another problem: some providers submit accurate photos of their front, and then a fake back shot.  The boob men aren't likely to care enough to point this out in the juicy details because it's not their thing.