The thinking behind this goes very much along with the line of thought in color photography and shooting video with regards to shooting with a flat picture profile. If you shoot in harsh, incredibly contrasty light, then you will have a whole lot less leeway when it comes time to process your images.
However, if you can wait for some slightly less contrasty light, or introduce some lighting of your own to help even out the exposure in an image this will go a long way towards giving you the latitude that you need in order to process an image exactly how you want it to be done. It is all about giving you options for when you get into processing mode. We can’t tell you how many times we have shot an image with one look in mind but ended up processing it a completely different way because we liked the look better.
This is so important with black and white portraiture because there are often things that you will notice or things that you will run into while post processing that you could have fixed if you shot to capture the most information, rather than shot to capture the desired look.
If you are shooting outdoors, then this can mean shooting earlier or later in the day when the light is lower and softer, and it can also mean shooting on darker, heavier overcast days. Your images will look lower contrast, muted, and dull right out of the camera – but that is what you want.
In the end, if you are set on your look and know it will not change, there is a lot to be said about shooting it as close to that look in camera as you can. But if you feel there is even a chance that you may like to tweak things, our tip is to shoot in lower contrast light.