Here are some other scenarios:
Spa Therapists: Tipping 15 or 20 percent is common practice in the United States but rare at any spa abroad, where a service charge is typically added to the cost of the treatment.
Concierges: Situational. If a concierge gets you theater or train tickets, or has been especially helpful during the course of your stay, then a $10 or $20 tip (in local currency) is in order.
Guides: Booking a ski guide to take you off-piste in Switzerland, a golf pro in Scotland, or a fishing guide in Ireland?All should be compensated in the 10 to 15 percent range, as they would be in the U.S.
A good tip on tipping?Get small bills or coins from your hotel’s front desk to make tipping easier. But if you have only large bills, it’s perfectly acceptable to ask a hotel porter or even a skycap for change when you give them a tip. After all, tips are a part of their business. They’re not embarrassed, and you needn’t be either. And if you don’t have local currency, U.S. greenbacks can work just as well as a "thank you" in London, Lombok, or Lahore.