ok heres the deal...changing the needle position changes the mixture throughout the rpm range with exception to wide open throttle. Say you are running rich at lower speeds, but are running right where you want to be at WOT. well that is where you would want to change needle position. Raise the clip position to lower the needle into the main jet further, thus leaning the mixture out. At WOT the needle is no longer obstructing the main jet so the jet is all that determines WOT mixture.
On the other hand if you are running rich or lean at WOT then you should be looking for a main jet size change.
Hope this helps...also may want to check your owners manual, or local speed shop for more info