If you have it on your PSU and your motherboard has a receptacle for it, then definitely use it. If one or the other don't apply, then obviously you can't. For motherboards that use the +12V as a source for the regulated core voltage, any additional conductors you can add to assist the ATX connector conductors is "money in the bank".
I actually got better results after unplugging the thing. Don't ask me for a rerasonable explanation on that one but its tested and true on my board. So far I am up to 235 FSB, and was previously stuck at 230 with the 4 plug connected.