To expand on what Chitown is trying to impart, the goal it isn't a matter of walking straight or putting one foot in front of the other per se, it is a goal of balance at the foul line. To have that, our slide foot should be in the center of the body when you slide under your belly button, not under your left shoulder. To do that, your slide step tends to be more "in front of" the next to last step, not off to the side of it.
What foot you use to measure where you're standing on the approach has nothing to do with your finishing position. Some bowlers drift left, some drift right and still end up with their weight properly centered over their slide foot at the finish position.
However, it is a bad idea for a right-handed bowler to drift right and for a left-handed bowler to drift left (excepting extreme inside lines). Doing so means that you are walking into your initial swing plan (ie. from pushaway to the top of the backswing) you will be forced to alter your swing plane or hit yourself in the back of leg.
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J.J. "Waterola Kid" Anderson, the bLowling King : Kill the back row
