This clip contains clear explanations of wheel alignment, toe, camber and caster.
It also recommends that you should get your car aligned after rotating the tires, which makes sense. But if the interval is pretty short (say 7,500 miles) then it's not very practical to keep up with that routine.
The more often a technician touches your car, the more risk there is of human error. And wheel alignment is, in my experience, one of the procedures that has a high chance of not getting done right the first time, resulting in having to bring the car back for corrections. It's hard to get it just right.
So unless the car pulls to one side or is imbalanced, or there is evidence of uneven tire wear I wouldn't worry about getting an alignment each time I rotate the tires. Also, be sure to account for the effects of road crowning when checking alignment.
