Hmmm... never even thought of using a dedicated coin for the purpose. I usually just use the first nickel, dime or quarter that I extract from my pocket. But maybe I'll start using that 1964 quarter I received in my change a couple of weeks ago (noticed it by the unique sound which I haven't heard in YEARS!)...
Or perhaps one of my three commemorative "Lynden Centennial" one-ouncers I bought back in '91...
This thread does open up the imagination. I could go on but since I'm a newcomer and board space is not infinite I'll stop here...