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This is what I do too... Although I always anticipate converting to b/w when processing the RAW, I find that it often makes for very good (better?) photos in colour at the end of the day. I suspect I'm probably not very good at the post-processing (for b/w - or for colour!) but the other thing that using a b/w preview gives me is a feeling for the form and structure (lines etc) within the photograph.
I always have the preview screen black and white as it helps get idea of what you will have at the end but shoot raw and convert it later. You have more control over the outcome with raw if you’re happy doing the work in post to get it where you want.