Anytime your bike is unattended outside of your own home, your bike should be locked. Even in areas where the risk of theft is low, bicycles are quick, easy, and quiet to steal away and you never know when you’ll encounter an enterprising thief. Even if you store your bikes outside at your home, they should be securely locked, and any bike left on a car rack should be locked. Think about how quick and easy it is to remove a bike from a rack and throw it in the back of a truck or van.
When you lock your bike to something, the lock can only possibly be as secure as what it is locked to. Wooden fences or small trees can easily be broken, and it is preferable to look for purpose built bike racks and tall poles. When locking your bike, you are looking for an immovable anchor point that cannot be broken. Small stop sign poles might look like a good option, but they are easy to remove from the ground or potentially lift a locked bike over, giving thieves the option to take the locked bike and remove the lock at their convenience in a private location.
Locking your bike in a well traveled area, ideally one you can see from where you will be while the bike is unattended, will help deter theft attempts as well. A thief stealing a bike from a dark, empty alley will have more time and options to break a lock than one on a crowded street.