Is there such a thing as "too much" chroma noise reduction? Should I just leave it set at 100% or is there an advantage to leaving some chroma noise in?
In our experience, 100% is fine, so we made it a default. Luminance noise looks much more natural than color noise so leaving some noise in Y and suppressing 100% of it in Cr/Cb is usually a good idea, unless you want to achieve some special effects.
The most important thing, of course, is to correctly build the noise profile. When the profile is accurate, 100% noise reduction in color channels is fine.