Ideally you use the same wavelength for densitometry you use for printing. With VC papers, this immediately creates a challenge because you're working with green and blue light. With pyro negatives, there will be a difference in density between the two. If you use a the green channel on a densitometer, you'll actually work with more density than you've read on the blue (high contrast) exposure. And vice versa: if you use the blue channel on a densitometer, you're working with less density than you think for the green exposure. It doesn't matter if the green and blue exposures (of the paper) happen at the same time. The error will still be there.
If you use a densitometer with an ortho response and that ortho response is indeed some combination of blue and green, the error should even out and you should be sort of OK for a mid-scale grade 2 exposure on VC paper. Unless you're working with a blue exposure only (i.e. only print at the highest grade), using the blue channel on a densitometer will be less representative for what the paper 'sees' than using an ortho densitometer measurement.
Using the UV channel on a densitometer will not be representative for VC printing; you'll always read more density than you're actually working with during the paper exposure. It's of course great if you print using a UV exposure as in alt. process printing.
Whether these errors matter much in the real world, depends. If the error is constant and systematic, which it'll likely be as long as you keep using the same densitometer and materials, you would 'automatically' compensate for it in the rest of your workflow. So I wouldn't lose any sleep over it anyway. Problems arise when you're trying to compare apples & oranges; e.g. in the article by Bob Herbst that
@drew tanner links to, the problem he encountered was that he was relying on visual density measurements for a UV-sensitive process in combination with stained negatives. Since density in the UV band tends to rise quite significantly as the wavelength of the light drops, this effect is quite significant and you're actually printing with substantially more density than you're measuring with the densitometer. In a visible light printing process employing a mix of blue + green light, and a densitometer that measures either green, blue or both, the error will be much less significant. It'll be there, but it won't necessarily be very dramatic or problematic.
TL;DR:
* Densitometry isn't an isolated process; it should be seen as part of the entire imaging chain and account for the printing (or scanning) method.
* It's easy to get lost in the woods of nitty gritty details, but much of the time, they're not a major problem from a practical viewpoint.
