This I don't agree with. I've found that I need 100% magnification to really see what's going on with my 1Ds3 files. That doesn't mean the display view is what will show in a letter size, or even 13*19 inch print - but that's not the point. Who knows, one day I may wish to repurpose a file for yet much larger sizes, and then what's really there could show in a print as it does magnified on the display. Also, for analytical purposes it is necessary to magnify the image to the extent needed to see what the file consists of. The only risk here is confusing pixellation with sharpening artifacts and noise, but I assume those of us doing this work are aware of that and know how to tell the difference.
Noise reduction is antagonistic towards sharpening and image details. We should, therefore, only reduce noise to the point that it is no longer a distracting visual artifact.
As George Bowen at Imagenomic told me today, "Pixel peeping at anything other than intended output size is a waste of time, especially in the noise reduction industry." Here's a .PDF from Noiseware on workflow:
As the PDF indicates, Print Size and 100% Magnification give two very different perceptions about noise. "You can drive yourself crazy trying to remove all noise from an image at higher screen magnifications, when you will achieve much better results at a view that corresponds to your output." [Pgs. 7-8]
George and I were chatting via e-mail because I am going to extend my reviews of noise reduction software and sharpening software.
There is no "one setting fits all resampling" setting for noise reduction software. You will almost certainly reduce noise too much by using higher magnification as your guide and obliterate fine details that could be preserved. "Be careful to not fall into the rut that some advocate of doing noise reduction at high magnification and then touting that the downward resized image will look even better. In all but minimal cases, the smaller image will lose a lot of detail using this method." [Pg. 8]
As with sharpening, your monitor is not a good guide for final sharpening settings. Prints can show noise that a monitor cannot reproduce. You need a hard proof.
You should always apply noise reduction on a layer because you might need to adjust the settings later. Tonal adjustments will affect the visibility of luminosity noise. Color adjustments will affect the visibility of color noise. You can find that your noise reduction settings were not optimal after you finish adjustments to color, tone, and sharpening. It helps to be able to go back and substitute other settings.
I'll close with this: "The key to attaining an optimum level of noise reduction is determined by your output type and size. There is no generic "one size fits all" approach that will allow optimum reduction for all of the various outputs that are available to an image. A small web viewed image will not need the same level of noise reduction as an 8x10 print, and the 8x10 print will require different reduction methods than will a 20x30 print. Furthermore, print output type, whether continuous tone or inkjet based, and paper types will play important roles." [Pg. 6]
The approach you propose is more efficient. Run noise reduction once to make a master file and then use that to resample into several different output sizes. As the "Noise Reduction Workflow Tutorial" makes very clear, efficient is not optimal in this case.
I invite you to ask Kent Christiansen at PictureCode (Noise Ninja) or Vlad at NeatImage. They'll likely give you the same advice.