I've only ever participated in one Mystery Quilt class and I HATED it. This was many years ago, in the early 1990's, and I didn't completely grasp what a mystery quilt was. I thought "mystery" meant that decisions would be made as we sewed and the outcome was unknown. Once I got far enough along to realize that I had no input whatsoever into the process other than rotely following orders, I quit. The black and fuschia half square triangles are a few of the pieces I actually sewed together.
I made the decision while piecing this orphan quilt together that I would lose the bottom of those mystery blocks. I could have added strips to make other parts of the quilt longer, but I chose wonkiness instead. I know that's not the decision that everyone would make, but I'm happy with it.
I thought you might enjoy a look at the back of the quilt. You can definitely see that I don't stretch out the back at all - I just try to get it flat and then layer and baste. I haven't hit any particular tucks or creases back there (yet), but won't bother me if I do.
I knew this quilt top already had a lot of seams on the front, so I bought wide unbleached muslin for the back. It has the advantage of showing up the quilting.