I don't have any professional opinions, but sometimes I think something looks good in sepia/b&w, then I process it, and it doesn't. I think it's up to your eye. If you have a group shot with a lot of detail, it depends on what kind of detail---certain things get can get lost in the midtones of b&w, so if you process it and it looks lost, then change it back! (Oh, the joys of digital!) Midtones can pop a lot more in b&w, and tend to change the mood, like you said.
If you have a landscape, for instance, where the grass is very detailed, you can process it b&w to make the grass stark, and give it a thoughtful, broody look. (Think about a lot of Ansel Adams' works.) If you have a dramatic skyline, it can look even more menacing and stormy in black and white. I shot the son of a boy from the back, walking down a pier at our local wildlife refuge a while back. Compositionally it was a good shot, but there wasn't much detail to it, or color, or a point of interest. To draw the eye away from the fact that it was a very busy picture (lots of sawgrass, swamp, the repetitive pattern of the wood) and he was almost lost in it, I converted it to sepia and distressed it, and all of a sudden, it was a pretty good shot. The effect drew your eye away from all the business and focused on Mark. You just never know.
Katrina and the other pros probably have a rule of thumb, but I would say because you are the artist, then you should process it to convey what you had in mind. I find myself playing around a lot with shots, selective coloring, etc., and sometimes I'm surprised with what I see.
Sarah
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