I personally never do. If I download a character I like I usually use them at 50% if I really like them other wise its 10 to 35% on a lot of different models.
Yeah I hear you about them having to be very good to keep. I've started going back and deleting some of my Renderosity G8 characters myself. But the Daz stuff I kinda keep the bundle ones. The non bundle ones I don't really download anymore.