All over the place. Definitely do not tip US cash if you are in the Eurozone to off the ship vendors. On a tour with 30 people from different nationalities, you might see people tipping nothing and others 20%. It's very difficult to give a hard rule. I would suggest to not tip under 5%. But it also depends on the type of excursion. If it is mainly a bus transfer to Florence with a guide sitting on the bus telling you something while you drive there and then you explore Florence on your own, I would tip less percentage than if someone took me on a 4 hour walking tour through Rome in the heat. I am sorry that this is so imprecise, but ultimately do what you think is appropriate, the people working in the tourist industry are used to everything and will definitely appreciate getting a tip over not getting a tip.