Last year my twist grip went tight and wasn't returning properly.

I removed it, cleaned and polished the bar under the throttle tube, probably adjusted the cable so there was just a little slack (I can't remember doing that but normally I would).

All seems to be ok...

I was browsing the web and read that TuneECU is necessary to adjust the throttle cable slack!?

If this is true can anyone explain why?

From memory, closed throttle leaves flies slightly open at tckover - TuneECU fully closes flies for correct cable adjustment.

Ah, thanks for that.
So if the revs aren't hanging and I'm happy with how it feels is there any reason to do anything about it?

I guess not - have adjusted throttle cable many times but not using Tune ECU.