Some people accumulate the extras as stash and use them to make scrappy quilts. Others prefer to give away all the scraps and buy fresh for the next quilt. I belong to the former camp, but sometimes I regret that choice, as those scraps can become burdensome. They take up space, they require some sort of organization, and they can make you feel guilty if you don't use them. On the other hand - well, they're fabric, aren't they? They make great quilts.
As for the extra blocks, you can always put them on the back of the quilt, with any of the matching scraps, so that the back has more interest.