It's important to use fabrics/battings as close to the same as what you will be using for your "real" projects. That way you get a better "feel" for what you are doing and come to know better what settings for your machine, etc.
I learned that for me, practicing, just for the sake of practicing, with the intent of pitching it, gave me too much freedom to do anything as though I didn't care, and I didn't learn well enough how to do real things! I had to switch to something that was a keeper ... even if it was not perfect FMQing.
Now, I try to make my practice pieces something useful ... placemats, grocery totes, tote bags, candlemats, etc. Right now, I am quilting a piece that will become a cover for a heating pad.