Some cases let you mount a psu in the bottom of the case with the fan either up or down .
Probably the extra airflow out of the case , with the fan up , helps cool the other components a little more , but causes the psu to run fractionally hotter
Do you have air vent on the case where you will install the PSU? Most cases have PSU air vent at the bottom, so its recommended to place the PSU fan facing towards the vent. However, You can place it with fan up also without any noticeable increase or decrease in temperatures. (Unless CPU fan/GPU fan/DC fan are blowing right on the PSU fan)