Tipping really varies by person, but here are some general guidelines that I think are common:
Full-Service Restaurants -- 15% for avg service; 20% for good service
Room Service -- This one varies widely....for daily maid service, some people tip $1/person/day; some tip daily; others leave the tip at the end of their stay. For a
DVC stay for a Family of 5 with nothing more than Trash/Towel service, I usually leave $10 at the end of the stay.
Room Service -- This varies depending on what they do.....bringing up a requested item, e.g. blender, side rails for a toddler's bed, etc., I usually tip $3. Coming up to fix something IMHO doesn't really deserve a tip as it shouldn't have been broken in the first place and most good service people wouldn't accept a tip. Bringing up food I think earns 15-20% just as a waiter/watress would do, but I'm not really sure about that one as I rarely order room service.
Bell Hops -- General policy is $1/bag. This gets pretty high though if you've gone to a grocery store and they packed your items in plastic bags which each hold 2-3 items, so in cases like that, I do some downward rounding. I'd go $1/bag if they were each large heavy bags or if they stored the frozen/cold items or if they did anything else 'above and beyond the call of duty.'
Pizza Delivery -- For a more expensive order (e.g. $30+), I'd probably tip 10%....for a less expensive order I might tip 15%. That's what I tip for relatively easy delivery....e.g., Pizza delivered to my front door at home which is within close driving distance of the Pizza place. If there were bad weather or if the delivery person had to make a long walk through a hotel, I'd tip more.
Valet Parking -- Seems some people tip $2 in and $2 out; others just tip about $2 when the car is retrieved. If it's a really nice car that you want well taken care of, a more significant tip is probably warranted.
Hope that helps! I'm sure you'll get lots of varying opinions!