In Mr. Leiser's book The Complete Book of Fly Tying he says on page 58
He goes on to say that because it wraps fine, tapers well and is easy to work with, it makes a good base for a quill body andFor a dry fly, I don't usually recommend using polypropylene or any other synthetic,.....
is just barely lighter than water. It will help, not hinder, the fly's buoyancy