Tyler Cowen
makes the case that it may be a good idea. How convincing this case is depends, I think, on how much faith you put in the marketing people working for book publishers. It's clear that their cover designs do provide some level of effective signals -- you can identify a book as "chick lit" that's unlikely to appeal to a male 26 year-old based on the cover -- but one has to wonder on how fine-grained a level this is going to work.
This article available online at:
http://www.theatlantic.com/politics/archive/2007/08/judging-a-book-by-its-cover/45985/