A bad stave for practice is about like saying a bad woman is a good practice wife.... Do your best to get it right the first time...
That being said, I am willing to bet a Christmas tree can make a good bow, but very specific conditions must be met, length, draw weight, length, knot placement, tiller, frontal profile, and others I havent thought of yet. I would say a Christmas tree bow is a good idea for a bored experienced bowyer, not a new guy. But if you wanna try it, good luck, start a thread and im sure we will all try to help ya... Oh, wear safety glasses