We paid for our gratuities upfront. I believe that if you put them on your onboard account, then like us they will give you these little ticket things with labels for each person you're expected to tip like your stateroom attendant, waiter, and etc.
You can actually hand these tickets to the individuals, but it is not necessary. I called the information desk and asked them what we needed to do since we had forgotten to hand the tickets out. She said that it is in the system and that the appropriate people would get their money regardless.