try it & see how you like it- there will be people who will tell you NEVER use a sheet for backing- then there will be some who will tell you they do it all the time-
some people will NEVER use a polyester- or a poly blend- again- others will tell you they do it all the time
i can tell you- (sometimes) using a sheet makes it a bit more difficult to quilt- the weave is tighter than the quilting cottons we often use-
polyester wears well- it also does not soften up like cottons do- a 100% cotton will soften with each wash --- polyester will pretty much stay exactly like it is- so it's not as *snuggly*
but really- if you like the fabric- the color is right- and you want to give it a try- do so- it is your project & everyone has the right to use what they like/can afford/works for them
