It is entirely up to you, at least if you pre-pay then you do not have think about it again, but saying that it is nice to do at the end so that you can reward those that have given you great service.
With Celebrity there are 3 options - prepay them when you book, get them added to your onboard acct or pay out cash at the end of the cruise.
On a typical 10n cruise the grats are £55.35pp if you prepay them (£5.53pp pd). If you wait till on board then they are $10.50 so you really don't save anything waiting till you are on board IM.
Are you a gambler?.....To pay up front means that you will have paid the going rate for the period. If you consider that the dollar will fall in value against the pound in the future, and particularly when you are sailing, then hold off and gamble that being the case. If you believe that the going rate is going to go the other way or that the rate you are being offered is a good one then pay now. As an extra thought and if you have the money to spare you might consider buying shares in Royal Caribbean they are currently at a good price of US$38.72. The 100 shares needed, so US$3872 worth, will give you a perk of US$250 onboard credit for a cruise of a reasonable length, reducing for shorter cruises. So a good return on your money if you intend to cruise more than once a year with them, although low risk it is still a gamble...Nei.
Just returned from a Celebrity Cruise, and in my view paying the gratuities in advance depends upon your budget. Drinks are not cheap on these cruises, especially with the 15% gratuity added on every time, even if you buy at the bar. If your budget is tight, it might be a good thought to know that you have already settled that bill, as the service was superb. You can always give a little more on board if you feel the person concerned deserves it.
Thanks - your advice has really been helpful. Princess add grats daily but we still tipped our waiters and cabin assistant on top of that as they really worked hard for us and deserved it.