[eluser]kanjimaster[/eluser]
As InsiteFX says, avoid underscores, but as to whether to use camel casing or hyphens, it makes little difference, just choose whichever you prefer and stick to it, or go with whatever the authors of any javascript library you're using do, or do the opposite so that you can tell your ids and classes from those that the library inserts.
There's not a single right answer to this, other than once you've chosen, be consistent.
