I buy 90" or 108" when I can't find the print I want or a suitable alternative in 120" - I wish all of them were a standard 120" though.
Not just for big quilts - I use wide backing for everything I can. Three yards of 108" wide cut in quarters give you four 54" squares - very economical backing for four smallish quilts and you don't have to piece any of them.
It's a good size for laps, table runners, throws, toddler and crib quilts, but I would like to have an extra couple of inches on each side, sometimes.