We usually take around $20 for room service tips, plus enough to tip shuttle driver and stevedores at embarkation. We usually use room service for coffee for hubby in a.m., plus normally one meal when I get worn out and just don't have enough energy to go "out" to eat.
But, we take quite a bit more for tipping and cabs, etc. in ports. As said, you can take most of your cash in larger bills and exchange them as needed at the pursers desk. But, I am always surprised how many singles I spend in ports. I also like to make sure that we have cash in hand when we get off the ship at the end. Porters, shuttle or cab drivers.