I purchased a Firestone lifetime alignment contract the day I bought the truck new. The alignment was out, as usual with all my vehicle purchases. Since then I have had the tires rotated, a comprehensive vehicle check and the tires rotated after every oil change (DIY) at Firestone.
I am a full-time RVer, and all such work has never been done at the same Firestone store. As for me, anyone driving 6,000+ miles with no changes in the alignment has been driving on a smooth test track and not on today's highways and streets. No slight intended to anyone as I am only commenting on my personal experience.
My alignment has always had minor, and two more critical, adjustments in 50,000+ mi. Unless you believe that Firestone has some policy to fake alignment issues, which I don't, then for me I made the right decision. I don't want to wait till I see a tire issue as certain misalignments take a while to manifest themselves.
With my Coach, where 5,000-15,000mi/yr is normal, I have a 3 axle alignment performed every other year. Each of the eight 295/80 22.5 Michelins cost $850+, and I consider the alignment reasonable.
Others will have other ideas and rationales, and I respect that.