Paying 15% on food may not be unreasonable - paying someone 15% on overpriced wine (already marked up 4-5x retail price) I think is.

When it's suggested I tip I try to work out what a reasonable salary for that job in that location is and then work out what a per task tip rate might be suitable - not a precise science but help avoids the situation where you are being taken for a ride. At JFK for a porter to wheel luggage about 50 yards the forum experts elsewhere suggested a tip rate that conservatively worked out that my porter would make over 100,000 pounds a year.