I've always treated the freebies as I would paid books and only get the ones I think I'm (hopefully) going to enjoy reading. For anything I've really liked, I'll go and see what else the author has available to buy, and the name goes on my list of authors to check periodically for new stuff.
I have *far* more paid books than freebies
Edit: I read around 200 books a year
Edit 2: Had a look through my Calibre library:
18% are "promo" freebies
11% are out of copyright free classics