How long you plan on keeping your rig?
I bought mine new in '98, and I purchased a lifetime alignment service for $129 back then from Expert Tire (Firestone subsidarary). The service agreement came with no limits on how many times during the course of a year I can take it in either.
As I've altered my rig over the years, and messed around with the front end, I've taken in for a "free" alignment. It was really handy when I was messing with new contral arms in the early years, and I was trying to dial in caster.
Even now when I do some harder wheeling and I know I've beat up the front end some, I take in to get things check out.
Really convinient too because the place is just a few blocks from work. I can drop it off in the morning before work, then pick it up at lunch time. I can definately say I got my moneys worth out of my life time alignment, and I plan to keep my XJ for at least 10 more years. I just hope the place is still in business then....