It seems almost too obvious, but one time you may want to consider going to black and white is… when you see strong blacks and whites. Like in this shot I made at a recent wedding:

Strong blacks, strong whites, nice greys, and an off-centre composition. That makes this shot, wouldn’t you say?

Assignment: shoot one photo in black and white today. You can shoot in B/W in your camera or do it in Lightroom/Photoshop/etc.