actually it's pretty much a waste on Mortars. they already turn rifles into giblets. If i have nothing else to supervise i'll do a mortar, but my officer supervises everything from tanks to stuka's(for good measure-not sure if it actually improves anything) I even supervise HMG's if i get the chance basically if it's firing i'll supervise it. and Yes BAR's are effective on them, but thats why i watch him closely.
EDIT* New officer is dead after his fantastic 1st match back... damn flank speed cromwell
Man you should really retreat him sometimes.