My advice would depend on what type of aircraft you fly most of the time. When I fly P-47s, my primary ride, I always fly with manual trim. The aircraft is pretty stable and only requires small trim changes, for the most part, for any speed and/or maneuver. This allows me to make very aggressive vertical maneuvers without the combat trim getting the controls out of whack as I come over the top. If I'm flying a 109, for example, combat trim is always on, as that plane is very difficult for me to keep in trim, regardless of what type of flying I'm doing.
Ultimately, you'll know the aircraft is properly trimmed if you can take your hand off the stick, and the plane stays at the same attitude as when you had your hand on the stick. For example, during a climb, you'll know if you're in trim if, when you release the stick, the pitch attitude stays the same, you don't have any roll, and the ball is centered. If you're flying straight and level, release the stick, and if you remain straight and level, you're in trim. If she wants to fall off on a wing, adjust the aileron trim. If she wants to climb or descend, adjust the pitch trim, etc.
Another technique is to map one of your stick or throttle buttons as a combat trim toggle. Turn it on to automatically trim the aircraft, and then turn it off when desired.