For our Sail and Sign Card:
We always use a credit card. In all these years, we have never had a problem with that.
Yes, over the years, there has been a charge or two hit our Sail and Sign card that was not ours. We simply went to Guest Services and when it was not our name or our signature...they removed it. (since waiters are not scanning the card...just writing down (or punching in) a number....errors happen.
Yes, your tips for 4 people for 5 days will be $200.00.
We have cruised with our two boys for over 10 years in the same cabin (the Fantasy 8 times ) and have always had a great time.