In general, a flat belly is less likely to fret or take a bunch of set, but some woods and designs work well with a rounded belly also. It mostly depends on what wood you're using and what kind of bow design that you want to make. A round belly is the ticket for English longbows made of yew or other wood that can handle the round belly. Some woods, such as locust or hickory, I wouldn't try a rounded belly with. If you're making a flatbow, a flat or slightly crowned belly is usually the best way to go.
