We did this on Harmony last June. My wife wrote a quick note in them for the steward, waiter and assistant waiter along with an extra gratuity over the pre-paid ones. It was way less awkward then just trying to hand someone some cash. I also found that it was a good way to teach my daughters about gratuities as they got to hand out the cards. They even asked the waiter and assistant for a picture because they got to know them so well.