Hi Steve,
I think you have much more flexibility if you shoot in color and convert to black and white.
Keep in mind that when you convert color to black and white in Photoshop (for instance, by using 'desaturate') Photoshop uses 60% of the green channel, plus 30% red and 10% blue. This default may not be the best way to convert a particular image. If you look at the individual channels while the file is still in RGB, you are, in effect, looking at the a color image through a red, green or blue filter. (hit command-1, command-2 and command-3 to see the three channels, then hit the tilde (~) (it's under the escape key on the upper-left of the keyboard) to see the composite again. You can do a custom blend of two or more channels to get exactly the tonal range you want. This also gives you a lot of control over which colors appear dark or light, and lets you control how dark or light different colors will be as gray tones.
If you photograph fluffy white clouds in a bright blue sky, the sky in the blue channel may look almost pure white with little or no cloud detail, while the red channel will look almost black with dramatically bright white clouds; the green channel will be in-between. By blending these together in different combinations, you can create the exact tones you want. Green leaves will be nearly white in the green channel, very dark in the red channel. Skin tones will be very pale in the red channel - often you can get a kind of fashion-photography look, with little detail in the skin by using the red channel - you can easily get the eyes, nostrils and lips floating in a nearly-featureless face. So, by blending different percentages of different channels to get your bw image, you can, control whether two different colors contrast more or less with each other as gray tones in the final image.
You can do anything you could do shooting BW with color filters, but you can do it after the fact and without the need of actually using filters. If you photographed a red flower against a background of green leaves, you could make the red petals and green leaves both dark gray tones that nearly blend together, or you could eliminate red to make lighter leaves and darker flower petals, or use mostly red to darken the leaves and brighten the petals, depending on the result you want.
Keep in mind that you often lose contrast or 'snap' when you convert a color file to BW, so you often need to tweak it.
Here's a shot I did of a model I worked with - this was shot on Provia with an RTS III and the 1.4 85mm, scanned with a Polaroid Sprintscan 4000+, and converted to BW in Photoshop:
http://www.pixelp.com/jocelyn/gallery_a/page-09.html
Take care,
- Paul
