The following is from Trev who runs & owns Highpower (wizard of Nos)

You only need to worry about retarding the timing on n/a engines, as turbo engines run retarded from n/a figures and the cooling from the nitrous means you could actually advance the timing on a turbo or supercharged engine.
Regards
_________________
Trev The WIZARD of NOS

ps. I run a 150shot with no retard & advanced slightly for 200mph run, so Trev is correct as usual.
Rod