Aren't the stock coolers for the Prescotts not able to cool the CPU well enough? If this is the case than it might not be wise to enable smart-fan at all (assuming that you're using a Prescott, of course).
Well they aren't as good as some custom ones, but the stock is able to cool it well enough. Stock is pretty good these days, and will keep the Prescott within stable temperatures if everything was installed correctly and the system is running fine.