Since wynn is frequently replaced by W, even in scholarly editions which preserve other archaic letters, there's not really much typographic tradition to draw on (I'm assuming here, perhaps erroneously, that you're asking about modern whiteletter faces).

I'm by no means an authority on this matter, but I've taught courses in the history of English, so my aesthetics are probably not completely out to left field. I prefer wynn to sit on the baseline, but I think the most important consideration in designing a good wynn is to make sure it is readily distinguishable from both P and Thorn. Giving it a descender is one way of doing this, which might well be why this variant arose in the first place.

André