I use .png for everything that I've capped myself, because I use .png for the actual caps. If I'm using an image that someone else has capped as a .jpg (like the Tarty Doris pics), I'll generally just leave it as a .jpg, because the image quality is already a bit poor, so turning it into a .png at the icon stage doesn't seem to make much difference.
If I were using text, h/e, I would alwaysalways use .png.