Dave,
Restaurants: I tend to tip fairly generously. 18-22%, depending on service. I tend to tip a little higher percentage-wise at the "cheaper" places - 20-ish% or so, and something around 18% for good service at an expensive place. But I vary that for service (I've gotten outstanding service at an inexpensive place for which the tip was north of 20%, and I've gotten rotten service at a very expensive place where I tipped at 15% or below).
If it's a "cafeteria" and not a "buffet", no tip. If it's a buffet and there is some service, such as fetching drinks, seating you, making sure you have what you need - I tip, usually in the 10-15% range. If it's a deal where it's a sit down restaurant and you choose between a buffet & menu order, I tip at 15% or so even if I choose the buffet.
If you get a bill that has "service included" or "service charge" applied, no tip is required. But if the service charge is 15% and you get exceptional service, I tend to leave a little extra.
Valet parking: I tip a buck or two depending on where it is and how fast they are with the car. 15-20 minutes to get the car and they're not too busy? I might not tip. At one hotel they mixed up my car with someone else. Oops. No tip. Prompt response, holding the door open, no evidence of joy-ride, etc. I tip at $2+.
As for the Matre d'Hotel. In some places it's expected. Vegas, for example, the M-d tends to have their hands out. Greasing palms will get you a better table. Don't know about Hawaii - my test is to ask for a better table and watch the reaction for expectations. The best way to handle it is to ask for seating in a particular area when you make reservations: "may I have a table by the windows". Note that some restaurants (the "power-broker" restaurants in DC, NY, Chicagah, etc) tend to have "regular" tables for their "regular" high-profile customers. Those are places where tipping won't generally help. In Germany, I found a number of resorts had "reserved" tables for their guests - the same table was "thiers" every day. Tipping is pointless.
At B&B and some full-service hotels, I'll leave a small tip for the housekeeper. Not everywhere, and not typically at chain hotels. Some resorts it's expected. A couple of bucks a day.
Tipping is expected for room service, bellhops, etc, and doormen that fetch taxis.
In much of Europe, the service charge is included in the restaurant bill. But some countries have local customs - for example, in parts of Switzerland the local custom is to round up the bill (which includes service) to the nearest 5-10 SFr.
Many of the service folks are underpaid since the minimum wage can be lower if tipping is expected. Terrible system.
Hope that helps.
