I went with the lifetime alignment deal at my local Firestone. Can't remember all the fine print, but i'm pretty sure its simply as long as I own the vehicle. Being i've just about have her paid off and plan on keeping her for a long, long time (god willing) it was well worth the $157 to never have to pay for an alignment again.
You'll have to do a Google search to see if you have a local FS near you. Your not always going to get the most top notch mechanics working there, but as I pointed out above, you can have it done as long as you own the vehicle so if you're not satisfied just take it back to them and one of the other mechanics run the diagnostics again.
Someone else on here also mentioned another lifetime deal that was similar to FS's. Can't remember exactly who it was (maybe Goodyear or maybe Tire Rack, not sure), but if you do a thread title search for alignment you might be able to find the thread, it was only about 4 months ago or so.