Anything goes!!
If you want to have the quilting show, yes, a plain back is better ... though it does not have to be light. You can choose a thread to help it show.
If you want to disguise the quilting on the back (eg. it doesn't always look good from the back!) ... a print will help to do the trick! Large, medium or small ... your choice!
And too, some use leftovers from the front, to create a scrappy back. Or a few extra blocks leftover from the front worked into the back can be nice!
It's really a matter of what you want and prefer ... like most of quilting, there's no rules!