*Pfft...* That's an easy one. He is called the "King of Pop," isn't he?
Groups that either change their primary genre or simply play lots of hard-to-define genres are the tougher choices. Tangerine Dream, for example, could fit in a bunch of areas (from some of the best acid music, to film scores, to EDM, to some of the best soaring guitar, etc.).
The main benefit I've noticed that a computer-based media collection has over a physical one is that I CAN put Michael Jackson into more than one place (dance, soul, pop for example). So when I can't decide if I should put something in "A" or "B," I just put it in "A" AND in "B."
I don't know if you do this in your record store, but I remember one down in Southern California that had these record-sized pieces of white cardboard and would use them in alternate areas. Using your example, they would have one in the Soul area that would say "Looking for Michael Jackson? Check the Pop area!"