I've only done 1 on the Magic. The account was automatically charged, no one brought any kind of slip to me during the event, so I didn't even think of tipping...not that I could think by that point anyways.
There was a tip added and I believe I tipped another $10 cash for 2 of us. The bartender/instructor was great with the class so I tipped more. If it was average I would have just gone with the tip that was included.
I'm pretty sure I tip and I'm pretty sure that's because we are given a slip at the end to sign. But usually by the end of the tastings I'm not remembering a whole heck of a lot beyond how to get back to my stateroom so I may be mistaken. So even if that's correct, I really can't remember how much. Probably 10-15% rounded up. Or more. Or something. I always finish all of the drinks so it's not just my memory that's not fully functioning by the end.