I do what you do -- work on one quilt at a time.....or at least one quilt TOP at a time.....and buy the backing for each as I come to it, which means when the top is done. I'm trying now to decide on backing for a Lone Star flimsy, mostly pinks and reds, made by my cousin, that I was given to finish. I occasionally piece a back but don't want to for this top.
So, yes, I agonize.