It's a big no-no, walking with a nocked arrow, and sets a bad example to youngsters and newbies as well, who look up to you. And someone is always looking.
I've killed quite a few critters from the ground, and like Thomas says, it's unnecessary. Look at it this way. The attention you have to pay to keeping yourself or friends from getting "stuck" by your arrow, or from dulling it in the brush or avoiding obstacles by having that 30 inches of projection sticking out from your bow, could be put to better use in spotting your game before it spots you! When a shot is imminent, then and only then should you nock an arrow.