Here is Viking's tipping policy:
Our onboard staff is dedicated to providing you with the best cruise experience possible, and it is customary to give gratuities in recognition of service. How much you choose to tip is a personal matter and completely at your discretion. To save you worry over whom to tip and how much, we automatically add a discretionary hotel and dining charge of $15.00 per guest, per day to your shipboard account, which appears on your final invoice at the end of your cruise. This charge will be shared among the onboard staff who helped support and provide your cruise experience, including the waitstaff, stateroom stewards, buffet stewards, galley staff, laundry staff and others. (Please note this charge excludes bar gratuity; a 15% gratuity is automatically added to bar, beverage, wine and deck service tabs.) Should you wish to make changes to the gratuity amounts or pay in cash, just stop by our Explorers’ Desk at any time. You may also wish to tip your local excursion guides or driver; we suggest doing so in cash at the end of each tour.
From Trip Advisor:
In Italy service, which usually ranges from 1 to 3 euros depending on the restaurant, is automatically added to the check and must be visible on the menu. "Coperto," the charge for the tablecloth, silverware, etc., is illegal in Lazio but may be added in other regions.
Normally, just round up the bill, a few Euro. If you were given an outstanding service, a good tip -- 10 euro in cash -- will make the staff happy, but you are not "compelled" to do so.
Tipping cab drivers is unusual, but appreciated -- especially if they help you with your luggage and provide you with useful info about getting around in that particular place.
Tipping hotel porters is appreciated. Same for the hotel concierge if he/she is helpful in making your stay easier or more pleasant.