the 'not for backing' goes back to our handquilting days, good quilting cotton has 60-75 threads per inch, bedsheets start at 250-300 per inch and go up to 1000 threads per inch. Very hard to hand quilt through...my first quilt had a sheet back
With machine quilting I don't think it matters at all anymore, but the old 'rules' get rrepeated.
and cotton percale is a type of weave that made it so sheets didn't need to be pressed if that was your thing, but now it doesn't necessarily mean 100% cotton, some, instead of a special weave will inclue man-made fabrics to keep them unwrinklie. Read labels!