There are not any true “rules” in photography, except that if you open up one f-stop you have to compensate by changing your iso or shutter speed to maintain equivalent exposure. Everything else is really a subjective decision. I started photography with a 35mm SLR, and because I was young and poor, I developed and printed my own film (at that time there was no such thing as digital capture). I spent the first 5-6 years working with black and white film, and learned much about tone and light, you see, when you photograph in monochrome, color will not act as a distraction from your images, but it will also not add any interest. It is a little bit of a double edge sword. You have to have strong composition and light. However, with black and white, you are working with a strong departure from reality. Take an image of a rose, when we talk about a rose, the first thing you probably think about is RED. But, when we photograph a rose in black and white, we are striping that stereo-type and the image becomes a study of line and from, texture, and other such subtle ideas that can otherwise be lost because of the color.
Part of my process for image making is pre-visualization. Before I take the image, I have already decided if it will be a color image or a black and white image. In my mind’s eye, I have already set the contrast that I want, where the tones will be placed, etc. Since I came from a film background (and I really didn’t have any extra $$$) I still tend to shoot conservatively, planning each image – shooting less, planning/visualizing more. For this, I spend much less time in Photoshop.