Denoise footage before keying?

Green screen implies a very controlled environment, with plenty of lighting.

Why then the noise?

denoising before does help, but remember, you don't have to keep the overly denoised matte. you can use your newly created alpha matte to refine your final denoise with just the actor minus the screen. There's a lot of other tricks too like normalizing your colors so there's less variation in color/luma so you don't have to use multiple keyers.

Remember, it can look like crap for colors, as long as you have a clean matte. then go back since its actually easier to ignore spill color in the beginning. This is why I recommend intermediate keying not final keying as that actually makes it harder to key.

I would speculate that it's better to denoise first. I haven't used the Red Giant denoiser, but if it wants you to take a noise sample the way Neat does, a large swath of solid green should make a good area for noise sampling. Then again, it might provide a distorted noise profile because of the high signal in the green channel and low signal in the red and blue.

However... you still have the non-green portions of your image for sampling. Thus there's still no disadvantage to doing it before keying.

Also, the denoising process should smooth out your greenscreen.

I can't think of any advantage to doing it after the key.