>>To avoid the exchange costs, bring the tip money in dollars and set aside. Viking accepts dollars or Euros. If you put the tip on your credit card (as you can) it will be in Euros.<<
IMHO, it's unfair to nickel-and-dime the crew by making them pay the exchange costs on tips. What's more, tipping in dollars makes the passenger look like a rube. (Can you imagine the reaction a European would get if he tried tipping in euros in New York, Chicago, or Denver?)
I was on a Viking River Criuse recently, and I was amazed by how many passengers tipped even the local guides in dollars (that is, if they remembered to tip at all). Is it that hard to grasp the concept of "When in Rome, tip as the Romans do"?