As much as I like Rockbox, it's probably not the ideal chiptune player. On my Sansa Fuze, SPC playback used to cause all kinds of glitches and crashes, and the noise channel on a lot of NSFs sounded kinda messed up.As for SIDs, well, I've never tried them. As far as I know, those are the only chiptune formats rockbox supports.
EDIT: Oh, I suppose I should mention that you can combine Rockbox's GB emulator with gbs2gb, you can play most GB / GBC soundtracks.