In Mr. Leiser's book The Complete Book of Fly Tying he says on page 58
For a dry fly, I don't usually recommend using polypropylene or any other synthetic,.....
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 and
is just barely lighter than water. It will help, not hinder, the fly's buoyancy