I save them for the opportune moment. For instance, I posted some pics awhile back of a star block I made for a potholder - but it ended up being too large...so I added a couple borders, took apart a railfence top I had that was too small, slapped the star block in between and tada - a lapsized quilt that doesn't look too shabby.
The way I figure it, you never know when those extra blocks will come in handy. Maybe sort them according to color and eventually you'll have enough of the same color to make a little quilt. :)