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I have often, though I suspect not enough to make a significant difference to someone who is already OK with the slight variance between un-worn coins.

I've seen enough wear to prevent them to be calibration material at least.

Depends on what your tolerances are. If you only need to be within a mm a coin is going to beat that by an order of magnitude.

We use a pack of cigarettes as a gauge for one of the jobs we do. Quick, (not so) cheap, and readily available. May have to standardize on a vape though in the near future.

Only on counterfeit £1 coins, before the coins were redesigned to make them harder to fake

I have coins that originally had milled edges that are now completely smooth.