Composing a tune in an assembler should be the entry exam to calling yourself a "chiptune" composer.
When I wrote my (rather ropey) C64 drivers, I studied Rob's code in a disassembler for a long time, figuring out how he did it. Then I pretty much copied what he'd done, like most other music programmers back then. Later on I made myself a little instrument editor which let me tweak sound parameters and dump the numbers out to include in the assembly file.
I did all my NES music in an assembler in the same way.
cTrix - I think the most impressive C64 music routine in terms of features and efficiency is Geir Tjelta's "Macroplayer". Don't know if he ever released it but he sent me one of Jeroen Tel's old tunes that he'd converted to use the Macroplayer and it was using about 9 scan lines or something mental like that.
Great thread Mat