I had exactly the same pattern with my ereader as with my ipod touch. At first I actively seek out freebies and download any old rubbish, but after a few months reach a saturation point - probably about 3-400 in both cases - when I just stop looking, and only download something if it really sounds like something I want.
There's no point hoarding anything that's permanently free. It'll still be there when you want to read it. It's those sneaky one-day specials that need a bit of consideration.
It's not just freebies. I've pretty much been the same with low-priced items all my life. It's just a bit easier with physical objects to tell when it's time to stop.