My rules for clothing are different than for quilted projects. When you quilt, you are essentially stabilizing the pieced portion with both the batt and the backing. If you quilt heavily, as I do, you will have some shrinking after washing, but it will be quite even and not distort the finished project.
I almost never do garment sewing anymore, but when I did, I always, always prewash. Size does matter with garments, and I wanted fabrics to declare themselves before I went to the trouble of making an outfit. Also, very big appliques on a garment? That would be a big red flag to proceed with caution and think carefully about technique chosen. I'd probably want to stabilize appliques on a garment, with a fairly stout interfacing or something on the back. But honestly, I've never put appliques on a skirt, so take Beairisgay's advice there.