One thing my recruiter told me is that a letter from your congressman that reads like "I'm writing to recommend _____ _ ______ for _____. I'm sure _____ _ ______ is a fine citizen and would be excellent for whatever he's applying for. ..." (He didn't say that exactly, but he did say to be sure it wasn't a form letter) In my case, I'd met my congressman several times and my sister had helped in his office, so I had an in there. I also tried one of my senators, but his office wasn't interested in helping me and I hadn't been around him much. Anyway, I would agree that you need to consider people you know and who can speak for your character first. I would have loved to have a letter from a former F-14 pilot who was convinced that I would be the best thing for the Navy since Admiral Nimitz, but I didn't know anyone like that. I'm sure they can usually tell if you're really reaching with your references; they've seen enough to know when congressman X is the type who lets his office turn out as many form letters as they feel like doing. That being said, I think it's probably not a bad thing if you throw in a letter or two from people like a congressman who doesn't necessarily know you, as long it's not just a from letter and as long as you have three or more from people who know you well. If it's one of the three required and it looks cookie-cutter, then it doesn't look so good, but if it's the forth or fifth (or beyond), then it could be taken as extra effort on your part even if it isn't that strong a letter.
Basically, as has been said several times here, make sure you get letters from people who know you (and respect you; another consideration about getting letters from people you're not close to is that it's harder to tell what they're going to say). Once you have at least three letters from such people, I think then it's ok to think about picking up a name or two.