The very first opportunity quilt my guild did was a yo-yo quilt. That baby was HUGE & yes, we did back it with a sheet. Ours had a very pastel-ish/30's fabric look to it, so they used a white sheet on the back.
My Dad won it at our quilt show, then donated to my aunt & uncle who used it to raise money for the Huntington's Disease Association. The first person to win it from them donated it back & they sold MORE tickets on it! It made a lot of money for them. :D