A while ago I was asked about the Luminance Noise slider in ACR - any "rules of thumb" for adjusting it ...
Basically, as with many things in PP, it's a delicate balancing act ... you want that slider high enough to soften the unwanted grain, but not so high that it adversely affects desirable detail.
Of course you should only ever adjust while viewing at 100% - in fact, that applies to all the sliders in that Detail tab.
It's a little tricky, of course, because while the bulk of the noise we want to remove is in the shadow areas, the detail we want to preserve tends to be in the highlight areas. This means we often need to scroll around the image quite a bit while evaluating our settings. If you want to be ultra-nerdy about it, you can develop your raw file twice - once with NR settings for highlights, and once for shadows - then blend in Photoshop with a luminosity mask. But even I don't do that very often, and I am Lord of the Nerds.
No discussion about noise reduction would be complete without stopping for an oft-neglected reality check: Noise doesn't matter as much as we think it does! We (well, I) get so bogged down in pixel-peeping, and worrying about stuff that honestly won't be visible in print. Very often noise isn't visible in print even to the critical eye of the photographer who printed it; do you think your misty-eyed customer is going to notice insignificant noise around the smiling features of their beloved? Of course not :)
Furthermore, we should discuss the relationship between sharpening and noise reduction.
If you carefully remove a lot of noise from your image, but then are ham-fisted with your sharpening, it is possible to re-introduce the problem. In fact, in ACR, it's possible to be reducing and exaggerating noise at exactly the same time (in the Detail tab) - not a great idea, obviously.
When sharpening in Photoshop (which is where most of your sharpening will be performed), you need to be very careful with the Threshold setting in Unsharp Mask (discussed further here.)
In very difficult cases, even more careful sharpening methods should be employed. I believe Smart Sharpen is good at protecting noise, but I admit it's something I've never used. I have utilised High Pass sharpening with surface blur to good effect, when trying to sharpen a noisy image.