I stitch twice across that corner with 1/2" between the stitching lines, cut between them and save the HST (half square triangles) which are the "leftovers" ... I am not sure how small the triangles are that I have saved, but I just stitch them together in 5 block rows (25 to the block) and then stitch the blocks together at a later time. Great for a scrappy baby quilt or as part of a sampler quilt.
I do it for exactly the reason that you have stated - - fabric is not cheap, and scraps are my favorite things to play with.
I put all of those HST into a shoe box size plastic bin ( I am SUPPOSED to make them into a quilt when the box is full, but I haven't had time to follow through, but there is always tomorrow, eh? ;-) ) <wave>