Offering a personal 'Thank you' to your cabin steward and wait staff at the conclusion of a cruise while handing them their gratuity is usually a very pleasant experience and far superior than doing your tipping via a computer using the pre-paid method. In addition, you may find that the quality of service might (repeat, might) not be as good if the staff serving you know that they have received your tips before the cruise actually starts!
The policy of mandatory pre-paying of gratuities is slowly being introduced as a means of preventing the practice (by some) of short changing or not tipping the staff at all. I think that the day will come, when gratuities will automatically be included in the price of the cruise by all cruise lines. Bobby