If you are happy with it then absolutely!
I have used sheets for backings and even piecing and have always been pleased with the results. The sheets have proven to be durable.
Some will say never, some will say yes. Do what you like. :)
I have heard that you don't want the really high thread count and that the sheets can be harder to hand quilt through.
Many places sell wide fabrics at a very reasonable cost. You may want to look around for that if you are not interested in using sheets.