If its a mandatory tip and you are on the dining plan then the receipt will list it as an item seperate from the meals - so you'll have a list of the items you have had and prices etc, the total, then the service charge/tip. When you come to "pay" using your Key to the world card you tell the CM how you want to pay the tip - so charge to room, pay there and then with a credit/debit card, or cash.
You can't just leave the tip on the table because when its a large party the tip isn't optional, its something that has to be paid so is "owed", but you can do it when you use your dining plan at the end so shouldnt be and delay.
We normally use cash because I imagine the tip system works like it does in the UK - staff get the tip in cash and it is theirs, but if its through a card then it gets paid through payroll and taxed. Maybe not but we prefer to do it that way.