The question was asked: What I use for value sketches?

I actually use several mediums. I use felt tip pens, fine tipped pens, 4B pencil and charcoal.

Sometimes mixed together and sometime alone. If I start a field sketch with say a felt tip or a fine tip, I may enhance that back in the studio with shadow and highlights, or I may use that sketch as a reference to start a fresh one.

I recently bought some water brushes to experiment with. Haven't used them as yet though.