I am a dyed-in-the-wool flinter and don't have a lot to do with cappers.
A friend uses CCI # 11 caps on his percussion and was having a problem with consistent double strikes, so I slightly reduced the OD of the nipple just enough to allow reliable full seating. It likely took no more than a thou or two. He brought along a tin of Remington # 10 caps and, although they appear to be about the same diameter, I was surprised to see they are actually deeper than the CCI # 11s. I was under the impression # 10s are shallower than # 11s. If I had fitted the nipple to the Remingtons, the taper would have been a little longer and the CCIs might have been a tad loose.
Seems to me, capper shooters would be well advised to find a brand and size that fits their nipple and stay with it or fit the nipple to their cap of choice.
Then again, I am just an old flinter, so what do I know about cappers