totally speaking out of my rear here, but I would think it depends on the machine

the early compacts are designed for convection (cooling) airflow, adding a fan might disturb that balance (that they did spend time fussing over to get it right), so while your cpu might be nice n cool, you could be devoid air from another part of the computer that needs it. Maybe a passive heat sink would be better in that situation? (ie its rumored that when they were working on the //c adding a fan did not do much to the system temp because the floppy drive was now running hot)

More details about which mac your talking about and wisdom from the forum will be more informative