i decided to go with the lifetime firestone alignment for $139.99 (on special). If i do happen to change my ride height, its not going to be by much so they wont notice, and i asked if the warranty was still valid if i changed tires and they said yes so it jumped on the deal. been driving for a week with mad camber in the rear so its nice to finally get thing straightened out.
i'll probably get an alignment twice a year. when i put my stock rims/tire back on for the winter, i'll get one and then another one when i put my 18's back on in the spring. other than that, i think once a month is overkill unless you're doing some track driving where the extreme situation "could" cause the alignment to go out of wack a little but for a daily driver, it should be fine once or twice a year.