Hi Joan:
If you have an heirloom quilt that is 100 years old DON'T "fix" it with a 70's Daisy print blended fabric sheet! We saw just such a thing at a quilt show and it almost made the judges cry that this very old quilt was pretty much ruined by the addition of that horrible backing sheet. I'm fairly new to quilting so I don't know much but I probably wouldn't shy away from 100% cotton sheets but I don't think I'd use a blend, ever. Like someone else said, No Quilt Police, use what works for you and enjoy your results.
Feathers