I’m not exactly sure what you mean by “trading territories.” Assuming, however, you’re talking about capturing a 1 IPC territory for the sole purpose of collecting that IPC while knowing your opponent will recapture that territory on his turn, I’d say that’s almost never a worthwhile trade.
From a purely economic standpoint, you’re sacrificing a 3 IPC infantry unit (at a minimum) to gain 1 IPC. Say your opponent has 1 INF in that territory, which you kill. You’ve just cost him 3 IPCs. On his turn, he counterattacks, killing your INF (thus costing you 3 IPC) and regaining that 1 IPC territory. In effect, both sides have lost 2 IPC, with no change in the status quo. With that in mind, you should ask yourself if there’s a better use for your infantry unit.
There are times when the territory in question has a strategic value much greater than the IPC value, and such a “trade” is in your best interests. In those cases, it’s simply a matter of calculating the number of hits you expect your opponent to get in the battle and sending in one more infantry unit than the enemy can be expected to kill. For example, 3 defending INF can be expected to get one hit per combat round. If you send in enough planes to ensure you’ll kill all 3 enemy INF, you only need to send 2 INF in to secure the territory.