Martin,
To specifically answer your original question, YES...it is a standard practice of mine to use cases that I'm fireforming for the first time as foulers during a match. Make certain that you have some way to keep them separate from your other cases. I usually use a permanent marker and put a mark on them somewhere so that I don't get them mixed up after firing...
It is not a silly idea at all...and something else I would mention to you is the distinct difference between a "fouling" shot and a "sighter" shot. "Fouling" shots are the one or two shots that you fire through your bore after cleaning your rifle to get your barrel back to the state that it will be in while you are shooting your group, and should NEVER be counted on for your true point of impact. Once you've "fouled" the bore, "sighter" shots are the shots you fire on the sighter target to indicate to you what your true point of impact is, and what the prevailing conditions are doing to your point of impact...
Good Luck to you!...