What's all the "worry" about???
You are going to be on vacation!! The tips are charged to your sail & sign card anyway ... that's less worrry to me
You guarantee the sail & sign card with either a credit card or cash .. or both
Relax ... enjoy the cruise don't overspend if you have to worry about the final bill, just add the tip factor in ahead of time. (and in the rare case that it should need adjusting, there won't be any problems)