That's what we were saying about positioning And for the most part it will probably keep firing on a squad/commander/large creature until it either dies or routs then switches targets (from my experiance).

There is also a % that units will actually be able to fire at the 'rearmost' and not 'mostly rear'. So while putting your commanders as the rearmost may seem like a good idea, it is in practice not. Unless you want to take the chance that a flier or archer to actually target the rear.