If you want Black and White *shoot* Black and White.
Spielberg didn't shoot Schindlers List in Color, just so he could desaturate it later on, never mind how much that would have cost.
He shot B+W to put himself in the proper frame of mind...
If you want the best B&W shoot a digital color (raw) original -- don't desaturate it, convert it. Amolitor is correct, digital cameras only record color (one exception), but that's not unfortunate. Derrel's suggestion of previewing a B&W JPEG is fine, but remember that's the camera preview.
If you shot b&w back in the day using film,
and you were serious, then you carried around at least a yellow, deep yellow, green, red and blue filter to alter the translation of color to grey tone. The filter color lightened itself and darkened it's opposite so a yellow filter would darken a blue sky and lighten a golden delicious apple as those colors were translated to grey.
(We all know Ansel Adams became famous for the decade shooting in Yosemite when his red filter got stuck on his lens and he couldn't get it off.)
Today with a digital color original you're not limited to using the yellow filter or the green filter; you can use both and throw in the red one as well. That additional control means better results; get the photo you want not just the photo you have to settle for.
(Spielberg had total control of the set, shooting out in the natural world we do not.)
Here's an example, one of my neighbor's homes:
I used Photoshop's channel mixer in this case on a 16 bit RGB file. When possible it's best to do the conversion to B&W in the raw converter, but in this case I wanted to be able to convert the image in discreet sections. Photoshop's channel mixer allows you to alter the proportion of each of the RGB channels as the file is converted. This is equivalent to using a red, green, or blue filter when taking the photo, or in the case of this image the red and green filters together (try that with b&w film).
The red channel makes the blue sky darker and anything red lighter. Use the red channel to separate the clouds from the sky. I converted the sky with 100% red channel. But in this photo the house siding is two shades of blue. I didn't want the house getting darker. And to make matters worse the house is ringed in blood red begonias and I didn't want them getting lighter not to mention the red brick drive way. Shooting film? Just use your graduated red to green filter.
So it's simple in Photoshop to apply different channel mixes to different sections of the photo. In this photo the sky and the two deep red hibiscus flowers were converted using 100% red channel while the rest of the photo was converted using 10% red, 85% green and 5% blue. A level of control that if old Ansel were here to see it he'd pee his pants.
Joe