First of all, IMO, tips have to be earned, that means that the service has to be good, tips are some kind of gratification for a good service offered. I will never give a tip to a person that does not serve me in an appropriate gentle manner, to put the dishes in my table, and come back to pick them up, is not an excuse valid enough to give a tip to anybody, that is part of their job, and they get paid for performing it...but any extra care, as asking if the food was good or not, to come once in a while to the table to see if something is needed, to refill the glass of water, etc...that means a good service IMO...I used to work in jobs that required to offer courtesies as part of them, and the owner always make us do it, that means for example, at a gass station you have to do the windows, check oil and transmission fluid etc, etc...just to place the hose in the tank, and come back to remove the hose, will never get a tip from me, that's their job and they get paid for that...
For the delivery guys I always give tips, usually the tips are around 10-15% of the total amount of the ticket as a general rule, some guys leave more other less, but that is the standard, I usually give 10% as is easier to calculate, when it is not an even amount, and round to the next number and period...