Strictly speaking, alignment has more to do with the adjustment of your vehicle's suspension, and not your tires. On the other hand, alignment problems can cause bad handling, uneven wear patterns and premature tire wear.
FUEL ECONOMY Poor alignment increases reistance. In their report on the Fuel Economy Impact Analysis of Reformulated Gasoline, the EPA figures show that bad wheel alignment could cost you 10% or more in fuel.
WHEN TO CHECK ALIGNMENT Most of the time you will notice a wear pattern developing on the outside edges of your tires. Also, if you notice a difference in handling - a pull to the left or right, for instance - that could indicate alignment issues. If you (or one of the kids!) run up a curb, you might also want to check out the alignment.