I agree that it's better to tip in euros or other local currency, but I think Viking's pre-trip information is what's misleading because it presents most of the tipping guidelines in U.S. dollars. If you tip $10 per day, you're actually tipping closer to 8.50 euros per day. Added up over the course of your cruise and multiplied by x number of passengers that can make a big difference for the crew. Their literature needs to be updated to account for the current rates of exchange. Also just on a practical level, U.S. dollars can't easily be re-used by the guides or crew unless they convert it to the local currency.