People are tipping less at restaurants than they have in at least six years, driven by fatigue over rising prices and growing prompts for tips at places where gratuities haven’t historically been expected. The average tip at full-service restaurants dropped to 19.3% for the three months that ended Sept. 30 and hasn’t budged much since. The decline highlights a bind restaurants find themselves in, as they face rising costs of ingredients and labor amid customer frustration over spiraling bills.