Retro-grouch is 100% correct.

Having lived in a city my whole life and now working as a bike courier I can tell you that the worst thing for your bike (when locked) is predictability and time. If you are in a store for 5 minutes, in a different spot everyday then even some of the absolutely cheap aluminum locks would be fine for almost any bike, I have co-workers who have consistently left their bikes unlocked as they run into buildings and have never seen them loose a bike.

On the other hand I have heard stories from people in mailrooms where they left their bike outside for a day with a NYC Kryptonite lock, locking skewers, and a cable lock and have come back later in the day to find the bike gone.

The only time you are truly "safe" is when the lock costs more then the bike, at that point no petty thief will want to take the time or energy to cut the lock, and a professional thief will recognize that it is not worth the investment.

Another word of wisdom: "I don't have to outrun the bear, I just have to outrun you" if you lock your bike around other bikes that are easier targets, you get safer.