Nowadays I mainly machine quilt (arthritis HAS set in, but not too bad yet - must keep knitting...): I find that good quality cotton or polycotton sheeting makes a really good backing. I have never put a quilt of mine into a show - all that competitiveness for a HOBBY just doesn't sit well on my shoulders - I would rather have fun, give my quilts as admired and appreciated presents, than go through the agony of some stranger picking over my work. For me it is not that important.
I would probably reiterate what others have said. If you are showing your work, avoid using them. If you are handquilting, be aware that they can be tough to get through. If you are machine quilting, or you have loads to get rid of, well why not use them?
In the past, our grandmothers used what they had to make warm and cosy bedcovers. If you look at older quilts, you often find a mis-match of weights and kinds of fabric, all thrown together to make a quilt or coverlet. Although we are going through some tough economic times at the moment, it is nothing to the kind of poverty many of our ancestors went through. However the same principles apply: using what you have and avoiding unneccessary waste, is an important principle to live by, and a great lesson to teach our children and grandchildren. If you have a sheet that is the right size and colour for your project, then my advice is to use it!