The norm in the Med is to simply round up. Unlike the cruise staff, these people (guides, drivers etc) do not require tips as the service charge is effectively built into the cost you are already paying.
Rick Steves has similar advice. He says: "Tipping in Europe isn't as automatic and generous as it is in the US"
For more details, see:
http://www.ricksteves.com/plan/tips/tipping.htm
Thanks everyone for all the helpful advice and for thinking of things I have not thought about yet!
