It depends; are you in a location with a lot of bike theft? I commute on a nice bike, only use one u-lock and have never had a problem with theft, but I try to keep my bike in visible locations, or under surveillance. Conversationally, a friend of mine had her rear wheel stolen while she ducked into a bakery to buy a loaf of bread. The bike was locked, but the rear wheel wasn't. It only took 5 minutes or so.