I've never heard the part about Carnival automatically charging your sign & sail card if you chose not to prepay your tips. I'm about to go on my first cruise (so I do not have any first hand experience) but it was explainied to meby a Carnival agent that you have the option of prepaying or using the envelope at the end of the trip. While I expect to tip at least the suggested amount at the end, I don't like the idea of having this decided for me. Just sounds weird to me that they'd charge the sign and sail card for something that should be left to the traveler's own discression.
I'd love to hear some feedback / verification about the sign & sail piece. thanks!