IMO your tip would be perfect to use on ANY FABRIC where the right vs wrong might cause you to waste time - not only in trying to determine which side to use, but also in the possibility of having to "frog sew". Worse yet, I know some people who would toss the block rather than rip. More waste!

A good idea. I have struggled with these quilt pieces and never thought to mark them. There are so many helpful hints on this site.

Now does muslin have a right and wrong side? Because I honestly cannot tell by touch or looking at it.