When making a practice block, I usually don't even use the fabric for the project! Just scraps that are handy - often all just muslin. I am a planned scrappy quilter, which means I will never run out of fabric. Though I have been known to piece scraps for big enough pieces. I cut as I go because changing my mind is part of the process for me. Perhaps I need more plum in the mix... that sort of thing. We all get to choose works works best for US.