This kind of thinking is wasteful. Every d20 has a finite lifespan. It was created, and it will, at some time in the future be destroyed, as all things are. That means it has a finite number of rolls in its lifetime, with an equal distribution of all possible outcomes. When you “practice roll” and get a nat 20, you have wasted one of the limited number of nat 20s that die has in it. Think of the 20s. Don’t practice roll.