Hello Steve and Maricel,
Yes you can pay your tips in advance. (on Canrnival). We will be cruising the Miracle this November, and we have already paid the tips. We did it by telling our travel agent to add it on to our final bill. Our statement indicates "Pre-paid Gratuities -- $80", per person. (It's an eight day cruise)
I believe that if you booked through the cruise line itself, you can pay the gratuities through the web site. I remember seeing a link there, but it would not work for me. I called Carnival. Turned out it wouldn't work because I'm booked with a TA. So I called her to add it on.
As for the crew, I've read elsewhere that yes, they know from the start that you have already paid, so they should not "expect" more. But more is always your option if the crew member went above and beyond the call, for you. We have done both before. Usually give a little more, when deserved. Once left it just as it was, because that was all he deserved.
Is is better to hand the tip directly? Well, I don't really think so. Because, as Mike said, the tip will be automatically charged to your account. In order for you to hand it directly, you will have to go to the purser's desk and have the tips removed from the account first. It is my understanding that any crew member who has had his tips removed by the passenger, will in most cases, be asked to explain "why ?" I think the goal of that is to determine if he did something, (or didn't do something that he should have done), to deserve "no tip". I would rather not put him in that position, even if I gave him a good tip later.