While there certainly is a tradition of 5 wraps, I've read more than one place that it makes sense to space the wraps in a way that suits the length of the area you're trying to cover. When you have sections in a salmon fly, sometimes tiers will only use three wraps in a section. The fly that graces the cover of "The Atlantic Salmon Fly" by Judith Dunham has six wraps showing, Jerry Doak has several flies in that book where the wraps number 6,7 or even 8 on his Black Dose. Mike Radencich says " The usual number of ribs wrapped around the body is five, but there can be more on a long shank fly such as a Dee or a Spey". I like 5 myself, I've always called it the "rule of 5". I try to use 5 on my wets, nymphs and Salmon flies. Don Bastian says it HAS to be 5 on wet flies. I wouldn't say it has to be, but for most flies it's fine, and traditional. I tie a # 20 flymph that 5 wraps is a little much on.
Eric