ATMs are widespread. Major credit cards are widely accepted but some smaller shops, trattorias and hotels might not take them.
Romans are not big tippers, but the following is a rough guide:
Taxis Optional, but most people round up to the nearest euro.
Restaurants Service (servizio) is generally included; if it's not, a euro or two is fine in pizzerias, no more than 10% in restaurants.
Bars Not necessary, although many people leave small change if drinking at the bar.
Hotels Tip porters about €5 at A-list hotels.