Tipping is ALWAYS a big topic.
Perhaps that is one reason we love Radison...er, Regent so much because we don't have to deal with it.

In general, we Americans over-tip in Europe. Not that the europeans object, mind you!
I think that for a good or ok tour, 10% is ok. If the guide went above and beyond, then clearly more is in order.

Just to give you a feel for how Europeans think of American tippers.....
Pre-ROAR we spent 2 days in Amsterdam..we had a wonderful time!
By law in Amsterdam, tax and tip is included in your total at a restaurant..by law!
However, the custom is that you round it up to the next euro for a small coffee shop sort of space & up to the next 5 euros for a good white table cloth restaurant.
The very nice restaurant that we ate at included a short note with the check.. it read something like the following: we are often asked about grautities. We recommend that if you were pleased with the service that you leave 10 to 15%.
I can only wonder if that message is delivered to non-english speaking diners.

All I can suggest is that you check out local tipping customs on the web before you go anywhere & then tip as your heart dictates.

Doug (who has over & under-tipped around the world)