For AC welding it is important to operate at a relatively high percent heat. This gives you the maximum efficiency and minimum off time during each weld cycle of the sine wave. So your weld schedule should be designed to optimize this situation as opposed to a high tap and a low setting where a large percentage of the time (each cycle) there is no current flow and the weld is actually cooling down.
The Tap Setting Rule of Thumb is:
The effect on the phase shift is shown below:
These four graphs demonstrate why a high % heat is desired for welding. One can see that as the current on time is shortened there is too much off time and the part cools dramatically during each weld cycle. Thus the reason for the given "Rule of Thumb".
Reterence: RWMA - Resistance Welding Manual, 4th Edition