Well, most of the people you can truly trust for alterations can also MAKE the garment they're altering - that's the best way to be sure they know what they're doing, how changes affect other areas of the garment, and so on.
That said, at least one of the tailors who has been mentioned specifically declines to do any alterations work (on things he did not make): Despos. Thus, if you're looking for alterationists ONLY, Despos would not qualify - even though he's certainly highly trustworthy and extremely high quality - it's just a question of what you're looking for.
Since you specifically mention alterations, and you also mentioned wanting someone from diverse areas of the country, here's one from Columbus, Ohio:
1053 Bethel Rd
Columbus, OH 43220
He does custom shirts, has a pretty nice fabric selection, has done tailored stuff in the past but no longer does from what I understand. From my understanding, his wife Marika is the only other sewer in the shop; she does the "easier stuff" like pants hems and so on, unless you specifically request Boris. I've had numerous alterations to very-high-end stuff (both shirts & tailored clothing) done there and have always been pleased, not just that it got done, but with the quality and attention to detail. Not cheap, but he's the only guy in Columbus I'd trust with the stuff that matters - I ruined a couple of shirts at other alterationists before I found him through a reference from the only good men's shop left in Columbus.