I typically make sample blocks because every once in a while, the cutting directions are wrong. Cutting out 30 2.5 inch strips can be a problem if they were supposed to be 2.25 inch strips. In addition, sometimes your color combo doesn't work as well as another color combo.
I sometimes join BOMs where you are automatically sent the kit every month. I make the first out of my stash. Once satisfied, I then make the block out of the BOM provided fabric. This way, I also get two quilts out of it (those BOMs cost a LOT of money!). One is for me and the other gifted.
When I make sample blocks, such as for a swap, I use the same pile of christmas fabric. That way, the blocks will work together into a new quilt.
If color is my issue, I put the sample block not used into an orphan block box. I periodically take blocks out of there for other projects, but if the box ever gets full, I'll just make an orphan block top.