Okay - I have another fabric purchase to report, but am also going to give myself a bit of an exemption in general when it comes to backings. I do have lots of fabric, but most of it is smaller pieces and so if I make a quilt that is bigger than a standard baby/lap quilt, I run into trouble.
For the last (string) quilt I made, I went through every piece of larger fabric I own and just couldn't find anything that really did it justice, and even though you don't see the backing 90% of the time, it has to work for me to be happy about the project as a whole. I went to the LQS to buy batting for it and decided to take a quick peek at what they had in widebacks (I'd already looked on their website and hadn't seen anything I liked), and lo and behold, there was a brand new (still in the plastic) wideback that works just beautifully. Of course, as the quilt is 64x80, I ended up buying a piece that will leave me with quite a lot of offcuts, but it's such a great fabric I'm sure I won't have any trouble making use of those extra bits in another project. Double (triple, if you count that I didn't forget the batting in my excitement over the wideback discovery) win!