Flying with down trim was one of the first things I was taught. Not only does it give your mind a constant reminder for which way to move the stick when you are low and inverted, but it is much more natural to release stick pressure to let a plane settle in, than it is to have to push the stick forward to get a plane to settle in. You can do the smoothest low level, high speed passes this way. Pattern and 3D flyers need to fly with neutral trim, but for flying a sport model with stall tendencies, down trim adds some built in insurance that you will keep the nose from getting too high when the airspeed is low.