Personal preference. For me I toggle it on to get a ballpark balanced setting at a given speed and then turn it back off again. I prefer a slight nose down trim. In the P-38 combat trim tends to want to force the nose up more than I'd like at slower speeds and I find myself fighting the stick to keep the nose low. I don't like it. I have all trim commands mapped to my stick which comes in handy in situations like when I lose that extra engine.
