If you want to opt out of the automatic tipping procedure, you can do so at the Purser's desk any time during the cruise. However, it is recommended by some one who has recently sailed on the Grand Princess that you should stop the auto-tip on the first day.
I don't think he will mind if I quote from his e-mail, "Canceling the auto tips was very easy, all it required was a visit to the Purser's desk on either deck 5 or 6. I suggest that you do this the first day on board to prevent several charges and credits on your account."
Hope this helps.
<font size=+2 color=blue>It's more than a cruise... It's duh LUUUUUV Boat!!</font>
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