most quilting cottons are 200 thread count. it is easy to find sheets that are also 200 thread count (or even 150-even looser)
batiks have a higher thread count than 'normal' quilting cottons.
if you buy a 800 thread count sheet you may have a problem but the higher the thread count also the higher the price, and since most people choose to use a sheet to save money i would think they are buying the lower thread count cotton sheets;
i do not buy into the DO NOT USE mind set. if it works for what you are doing, no harm done. but high thread count sheets may be more difficult to hand stitch through, sewing machines have no problems and as long as you use a new sharp needle there are no problems either