Short answer? Whenever you want to.
Photography as an art goes beyond the restriction of the fundamental tenets our photo teachers tell us to follow, like creating proper exposure, avoiding loss of details caused by clipping, following the rule of thirds, the golden ratio, etcetera, etcetera... Yes, these are rules, but the same rules can be bent - others broken - all for art's sake. The same principle goes to deciding whether a photo should be rendered in its natural colors, or you strip it off instead by pulling down the saturation levels on your photo editing software. Again, there are rules to follow (which we'll discuss after the break), but you decide what to do with them in the sole purpose of expressing the photo's message and visual impact however you see fit.
Of course, actions reap reactions, and there's always a chance that deviating from the norm might not fare well with your photo's final outcome. You've been warned.
Consider this photo:
Camera Settings: 1/250 sec. at f/8, ISO 100, at 90mm. focal length
This is a photo of my friend Chai who helped me out in setting up the lighting for a casual photoshoot. I needed an idea as to how the light would fall on the subject so I forced- err, asked her to pose a bit. I liked the color, the skin tones are quite nice, proper exposure was attained on parts of her face where it mattered... For me, the shot was perfect, and I'm happy with it.
But then boredom mixed with curiosity struck, and I decided to take the colors off and see what this would look like.
It looks okay, but it only looks okay. Her skin tone is somewhat similar to the background, and the overall emphasis is a bit dulled. I want a popping black-and-white photo, not a sinking black-and-grey(ish) one, so I adjusted the tones by bumping up the highlights and lights a bit (in Adobe Lightroom) to produce this final image:
Due to the tweak, her face is now accented from the rest of the image. I also adjusted the levels on her eyes and hair to make them pop a bit.
Here are all the images placed side for comparison.
From L-R: Colored (duh ^_^), B&W (untreated), B&W (enhanced)
With this we can now derive a couple of guidelines in choosing to desaturate any given image:
So go ahead, feel free to set the colors aside. It's all up to you anyway.
...just don't forget to create a back-up first in case the modified image bombs. ;)
AF-S Zoom-Nikkor 18-105mm. f/3.5-5.6G IF-ED VR Lens
JTL Versalight 250W and 360W Strobes
Pixel King TTL Flash Trigger Set