Depends. If the power supply has silent mode, in other words the fan can turn off until you are using a certain amount of power, then it is best with the fan pointing up. This is because with fan pointing down the psu has very limited ventilation. With the fan pointing up, however, it gets at least some ventilation from the rest of the PC. You would think it's worse because the air coming from the PC is warm but actually it's not. However if the case is badly ventilated, it's not going to be used on a carpet, or the PSU does not a fan off mode, or the PC has a shroud over the PSU, then it is better to face the PSU fan down. In most situations it actually makes not a lot of difference. The ones to be really careful about are shrouds that cover PSU and carpets that can block the air intake completely.