I looked around. It seems that teaching manuals vary (though all recommend being able to use both hands just in case). People who teach themselves tend to use their dominant hands. Whatever works

Stick appears to be left handed. He canes with his left hand. (Though again, Matt was probably taught at the hospital while god only knows where Stick learned)

Idk. Basically we just don't know, so this prompt still needs to be filled. Idk maybe something about mastering proper lefty writing, only to be blinded and never be seen writing again. Stick instructing him how to use it to his advantage, using weapons left handed, an AU where he starts his own store for left-handed supplies.