I think many folk would recommend it be way early in the process, probably before main adjustments (ACR/Develop or the non-Adobe equivalents). But I don't do that — I wait after the main adjustments.
However, I do it before sharpening. That's a hard rule for me — do not sharpen noise.
What I haven't decided: sometimes, if I feel that I need to use NIK/DXO Viveza after ACR, I DeNoise after that (on the newly Viveza-created layer). This is probably because I want to save the ACR layer as untouched in case I need to go back to that point in the workflow. So if I decided to move DeNoise to before Viveza, I would have another layer. (dup the ACR layer, DeNoise it, then feed it to Viveza which will create another layer). Does anyone have input on that sequence (would DeNoise before Viveza make sense)?
Chris