I periodically get all riled up about this topic. I've noticed in the last few years that yardage width is not included on bolt information as much any more, and so I've taken to having it measured before I have it cut and when I am calculating the yardage needed I base it on 40" WoF. Part of the reason I have the amount of small bits of fabric that I do is I'd rather have half a yard too much than be 4" too short.
Back around Y2K I used to do a lot of fabric swapping, and I was simply amazed at how often I could barely get (4) 10" squares out of yardage marked 44/45". I tried to start online indignation but was mostly met with apathy. I wrote letters to my state's weights and measures division as well as the federal. The quilt fabric industry is billions of dollars per year, it's not a minor thing.
The thing is that a lot of times it is NOT a by-product of the manufacturing process or shrinkage, it is done with the intent to save/make money for the manufacturers by skimming off a couple of inches to us, but on a 600-1000 yard length of greige goods (basically raw fabric) it adds up to a lot of extra yardage for the manufacturers. The way the printing process works they know very well what the size the rolls are and that we are being shorted. And then boo to the manufacturers who add a 1" unprinted selvedge to each side...
Related to backs in particular, boy oh boy, how often then are badly cut! I've lost significant yardage truing up the fabric, almost half a yard once because I had a bad cut going in one direction on one side and the other direction on the other side, I ended up having to buy another back. I've always included extra just in case but instead of inches I've started getting an extra half yard. (Edit: Sometimes it isn't so much the cutters that are the problem but that the fabric isn't folded/wound around the bolt correctly.)
So the answer is to always do a "dry fit" of the back to the top, no matter what the measurements say. I've always been a strong proponent for prewashing as well.
For my one little sparkle of good news, the quilt backs rarely go on sale at Joann Fabrics and so it's a great use for those 50-60% off coupons when the rest of the store is already marked down 30%
