Put it another old-fashioned way, Rob, you may take a good photo but not develop the film well - printing it is always going to be a struggle. It's the same with digital colour images going to B&W.
Thank you, John. I'm quite surprised that my innocent comment about making the best Color version I can before converting to B&W has stirred up such a commotion. What I actually try to do is very much like developing the film correctly.
I don't do a lot of fine tuning on the color version, except in the rare occasion when I think an image has significant potential in color. I just do the basic adjustments to make it look "pretty good." Then, if I want a B&W version (which is almost always based on my intention at the time of shooting,) I will make a copy that becomes the B&W version, where I do all sorts of fine tuning, including curves and anything else that I feel is needed.
My best color versions are given a yellow tag, and the best B&W versions are given a blue tag, so I can easily separate them.
It has never, ever, occurred to me to convert a B&W version back to color!
-Eric