When do you use black and white?

The title, pretty much. This is a topic that's been discussed basically ever since there's been an option other than monochrome, but I'm curious as to your own thought processes when choosing to shoot or make an image greyscale.

Do you head out with your camera set to black and white(or with a roll of b&w film) for an entire session, or do your black and white photos become so only in post? In either case, what pushes you towards that aesthetic direction?

Originally, I would set out to shoot only in black and white, but recently, most of that happens in post. Part of this is because I'm now a RAW shooter, meaning that photos are always saved in full color and need to be made b&w afterwards. So now, if I spot something I feel would look nice in b&w, I set my G3 to monochrome mode so that I can see the live view EVF preview in b&w, which can greatly affect how you see and compose a scene. Other times, I don't realize the benefit to monochrome until after the shot has been taken.

Practically speaking, b&w tends to look better than color on days that are cloudy or dull. Conversely, it often looks better when the light is too harsh. Finally, it also gives you a bit more room to mess around with in post. High ISO or heavily exposure-manipulated images, for example, tend to look better in b&w than in color. I tend to like high key photos, and rarely do I feel noise degrades my B&W photos when I'm increasing exposure by 2 stops in LR.

As for why I do it? To emphasize lines, shapes and textures, normally. While color is a great thing to have, oftentimes I feel there is an image lying below the surface that color tends to mask, similar to how you might crop out elements of a picture to keep the emphasis on what you want.What about you?