Just wanted to add a data point: At the HR Houston, after asking for my room number, the waiter presented a bill with a 18% service charge already added, and told me the bill was already taken care of and I just need to sign it. This really is how all Hyatt properties should go about it.

There is still space for a tip; I did add a couple extra dollars on one of the days when the service was particularly good, and that was also removed from my bill. The FD mentioned Diamonds are given a $24 credit for breakfast, and my total including the service charge and tip was less than $24; I don't know how it would work if the food portion of the bill were $24 and the service charge or any additional written-in tip were added on top of that.