I think Apple suggests that you discharge the battery once per month (I believe that Apple defines this as using 100% charge each month, though that doesn't have to be all at the same time -- ie, "use 50%, charge, use 20%, charge, use 30%, charge" would be a full discharge). I don't remember anything about whether leaving it plugged in is bad. I would think that it's the same as leaving a notebook plugged in all of the time. Unless someone else thinks otherwise, I would say, don't worry about it.
Edit: here is Apple's FAQ on iPod batteries. The gist is that my comments above are correct.
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Edit: here is Apple's FAQ on iPod batteries. The gist is that my comments above are correct.