is it safe to be behind a microwave while it's on?
A home grade (or dual use unit), unless defective beyond reason (eg if there is a hole in it or an interlock is broken) will be always safe from all directions, since any risk of harming persons or animals and/or creating radio interference (Microwave ovens share a frequency band with WiFI!) behind a wall or in the room below or above is unacceptable.

Same can be expected to apply to a commercial unit, unless it would be some very special device not intended to be used as a normal microwave oven (eg one meant to be operated on a conveyor), which can be expected to come with very clear warnings attached in that case.