Out of alignment wheels will decrease the gas mileage of your vehicle. They will also reduce the handling performance of the car, which makes it less safe to drive. And of course the lifetime of your tires will be shortened resulting in you spending more on replacement tires. When should you have the wheels aligned? Align your wheels after installing new tires or rotating the old ones. Have them aligned after replacing or having work done on the steering or suspension. Also go for an alignment after any collision repairs.
Keep your wheels in good alignment and you will save a percent or two on gas!