I'm also left handed for writing, but my right is dominant for practically everything else (ball sports, racquets, bats & clubs etc
)
I have found though, a left hander tends to be more dexterous with the non preferred hand, for example with racquet sports like tennis and squash i can play equally (as badly) with my left or right hand
When i used to play competition squash i would often change hands if it suited.
I have never actually felt a dominant side when training karate, I cant honestly say i prefer one side or the other, as for my spinning back kicks, well, they suck on both sides
I probably kick better with my right foot, but that is more to do with the fact that my right knee is completely stuffed, therefore when kicking with my left foot the dodgy knee has to bear all the weight and i dont move as well with it.
Interesting that all us normal people (left handers) can use the right hand better than the rightys can use their left
Bren