Best place to put Microwave Oven
Microwaves go in the kitchen.  I would suggest that it is up in a higher spot so kids can't play with them.  That is a good reason they are often over ovens along with the venting.  

Also from an electric point of view almost all microwaves require a 12A dedicated circuit.  For a kitchen you will probably have several runs like this and it is just bundled with those.  For a bedroom you would be basically running one circuit into a room for one outlet, where the rest of the room is on another.  Just a little odd.

From a safety stand-point microwaves draw a lot of current.  I guess there is more of a chance (albeit very low) that microwaves cause more fires due to current fluctuations than most other things.  I would much rather a fire in my kitchen than my bedroom or anyone's bedroom.  

And practically speaking I remember living in the dorm and my roommate burning popcorn in the microwave... Good luck falling asleep with burnt popcorn smell.