Back when I first started out with the P38, I always used combat trim. I got the "feel" of it. I liked it, and I had pretty good success. Later though, after years of experience, I find I like life better without it. I still start out with it on by default, and it's generally still active throughout my climbout and long commutes, but just as soon as I get into any action, I find myself reaching for those convenient trim controls at the bottom of my joystick (instantly de-activating combat trim). From then on, I can just "feel" the need for manual trim, and it now seems completely natural to use it as necessary whenever I can feel my joystick pressing against my wishes. It took me several years to get to this point, and I suspect that's the pattern with most folks. In the beginning, you probably won't want to mess with trim controls at all. Just let Combat Trim help you out. It's beneficial most of the time.
With experience, however, you'll gradually become aware that sometimes it is fighting you. At that point, you'll want to get a joystick that makes trim buttons convenient during the frenzied moments of flight. As you use them to disable Combat Trim, you will feel your aircraft becoming more "alive" in response to the ebb and flow of air over your wings. You'll learn to love manual trim.