You should never practice with stale coffee. The Co2 from fresh coffee will effect extraction and taste. Fresh coffee extraction is something that you will never stop learning. From understand when coffee peaks to how it taste. Also supermarket coffee cost just as much as fresh coffee. If you are using a blend and its fresh, its takes a good barista months to understand excatly what it does. From which days it peaks, to getting the most out of it on a particular time. Stale coffee is only good for one thing..... filling up a rubbish bin.