General tipping guidelines can vary, but in here are some suggestions I can offer from our traveling and experience.
Skycap - $1-$2 per bag (more if they're heavy)
Taxi, shuttle driver - 15%, 20% (or 15%+$1/bag) if driver helps with bags
Courtesy shuttle - $1-$2 per bag if driver helps with the bags.
Valet - $2 or $3 upon returning the car to you
Bellman - If helping with bags, $1-2 per bag. Same at checkout.
Room Service - 15-20% (but check to make sure it's not already added)
Maid service - $2-$3 per day, more if you make a big mess and they clean it up

: (Tip daily because you never know if it's going to be the same person everyday.)
Restaurant server - 15-20% before the tax. One important side note: If you have a coupon or get something comp'd, tip on the TRUE price of what you ordered, not the price you were charged. Ex: Meal is $15 (before tax for 2 drinks and 2 entrees), but you had a buy-one-get-one-entree-free coupon. If you had paid for the second entree the bill would have been $20. Tip should be based on $20, not $15.
Self-service restaurant or buffet - If the server keeps your drinks refilled and/or brings some of the food to you, 10%.
...As far as roomservice this is hardly a custom. It fairly uncommon now and almost non existent in generations before us.
Are you saying tipping is hardly custom and is fairly uncommon or ordering room service is?

I agree room service is more rare now than previously. I disagree that tipping for it is not common.
- Dreams