Optimal Level of Wavelet Decomposition for Denoising
I'd say it depends on the noise properties and of course the image itself.

What you can think is that most Denoise Filters can handle only the High Frequencies of the noise.
Hence the decomposition process moves Low Frequency of the noise to the High Frequency part for the spectrum.

So if your noise is white you need to go down as you can.
If it colored and most of its energy is in the High Frequency you can do only few levels.