Right and down thrust is basically a carryover from free flight days where an aircraft had to transition from a high power climb to a floating glide without any control input. The right thrust enabled the model to climb to the right and glide to the left. The trim was set for an efficient glide, which tended to cause excessive pitch up under power, even with a pretty far aft cg.

A radio controlled model with both control and trim on three axes really doesn’t “need” either. It is usually adequate to simply trim out any turning, or climbing tendency. As Rotaryphile said, a little down thrust is appropriate (though not essential) for trainers due to their forward cg and good pitch stability. When building a kit it’s good to put in the thrust offsets called for on the plans because experience with that particular design indicates that that requires minimum trim input. The only way to know for sure is to fly it and see. I have two original designs that I have built recently and both flew right off the board with no side or down thrust offset.

(Any thrust offset is only good at one speed and power setting. You still have to trim or hold control at every other flight condition)