I guess ergonomically speaking the best way to hit the spacebar would be to alternate between the right and left thumb so that one hand doesnt get more use than the other, but ever since ive started using the keyboard ive always used the right thumb, so using anything else seems a bit forced. Ive noticed on my abs spacebars that I tend to hit the bar a little right of center (between the 'b' and 'n' keys).