I don't see the need anymore. All I do is put a hue/saturation layer above everything, remove the saturation entirely and hide the layer. Then when I want to look at the greyscale ver I just turn on the layer for a bit.
Depends on the piece and style I'm going for. I prefer to paint in color since it gives me more control over color reflection and general color ambiance right from the get go, but sometimes I paint in black and white to get a more "gritty" or "dulled" color to my paintings... usually I only do this with textures and it works quite well.
Going from drawing with graphite to painting in Photoshop, I originally attempted starting with greyscale and adding the color after, but I found this technique to produce an effect that I wasn't looking for in my pieces. I'm also familiar with painting oil on canvas in the Bob Ross style, so painting other things was a small step out of my comfort zone. After some time of practicing with colors, I've managed to combine the techniques of graphite drawing and digital painting to produce some rather interesting works. I've also started painting photocopy portraits that are strikingly realistic, though I try to keep the look of a traditional painting.