The most common time to balance your automobile wheels is after they have been rotated. Automobile manufacturers recommend a method/schedule of rotation in the vehicle handbook, however, a good "rule of thumb" is to rotate and balance tires every 7,500 miles. This is a good "rule of thumb" because it is the time when most manufacturers recommend a service that checks out the car and also includes changing the oil and filter.
It's important to balance at least the tires on the front end of the vehicle is because the tires at the front and rear tend to wear unevenly. In fact, the tires at the rear tend to wear more flatly than those at the front at the front end. Also, the tires in the front are typically more sensitive to slight wear and out of balance tires.
You should consider having your front wheels balanced if you have a vibration through the steering wheel that starts about 50 mph, gets worse around 55 mph, and starts to fade at 60-70 mph.
Another time you should have you wheels balanced is after installing new tires. This is normally included in the total price of installing new tires. [This brings to mind something you should consider when buying tires... Good quality tires do not need much balancing. That means they do not take many ounces of weight to bring the wheel and tire assembly into balance, so they do not cause any "out of balance" issues.]
Wheel balancing prices vary depending on the type of wheels you have on the car. Most cars now have aluminum wheels that can require the technician to use wheel weights that have to be stuck to the rim. This can cost more than the tradition wheel weights that clip to the rim and can be installed in seconds rather than minutes.