From an employers standpoint, I doubt it would make much difference one way or another. In fact, with such a variety of living arrangements, I'm sure its a lot more common than you'd think, and the law firms or other potential employers might note your good sense in getting a reliable mail situation set up. Or, you could be right. But I think its a good idea to get the PO box, as long as its not costing you an arm and a leg.