Looking over a few books that I got to help me get started, I've noticed that most tunes don't have a tempo marking. Some books don't seem to mention tempo (at least not that I've noticed). One of my books (by our own Dan Levenson) suggests in the introductory material that 120 bpm is a good upper limit, and that many players stay lower than that, in the 100 to 108 bpm. But listening to recordings that I can find here or there, it seems like a lot of people are playing faster than that. I guess that begs two questions. First, are there pretty standard tempos that clawhammerers (or string bands, or fiddle players) often take these songs? Second, how much variation is there from song to song? Are there tunes that are traditionally played slower? Am I butchering a song if I take it too slow?
I don't really plan to set a metronome marking down on all my song books, but I do like practicing with one - it reminds me to keep my eighth notes honest, and it's been a big help for one or two places where it seems like a drop-thumb makes sense (I tend to panic and either hit them too fast, or miss because I tried to move my thumb too quickly).