I think for most of the FIRE crew, SS is irrelevant except as longevity insurance; if you're planning to check out of the workforce at 40, then you have to cover at a minimum 22 years before you're even eligible, and you probably want to wait for 30 to max those payments long-term. Which basically means saving about the same 'stache as you'd need without it (IIRC, the Trinity Study was based on a 30-year retirement period, so you need 25x costs just to get from 40 to 70).
We didn't conceive of FIRE when we started working, but we still based our savings rates on the assumption that we could rely only on what we could control ourselves -- so no SS, no pension, no inheritance, etc. As we get older, however, it is more likely that we will get something, and so we have begun to factor that in. Specifically, now that we are over 50, we belong to the category of "people whom politicians don't tend to want to fuck over because we have money and vote," which makes it very likely that we will get something; however, I do still assume it will not be the full present value of the benefits offered. For planning purposes, I generally assume that all the benefits will be taxed and that it will not keep up with "real" inflation, as those have historically been the most popular ways to trim SS benefits, because they hide the real impacts from the voting public.
Similarly, DH's company went to a cash-balance pension, and he is vested, so we know we are getting something there as well. For that, I tend to assume that we will get whatever the current balance is and don't count on continued employment, further contributions, etc.
But I still probably wouldn't FIRE if we needed SS/pension to meet our base level of expenses. I tend to plan in "buckets," so what I mean by that is that I am still not planning to run my "RE to 70" bucket down to zero based on the assumption that the pension and SS will then kick in and cover everything.