It really depends on the airflow. You typically want air to come into the case, pass over the hard drive(s), pass over the motherboard components, then the CPU heat sink, then exit the case. Since most PSUs have fans oriented to vent air out, exhausting the CPU air directly through the PSU is acceptable in that configuration. Unless your CPU tends to get very hot, in which case you probably want to vent CPU air directly out of the case using a duct. That way the PSU only gets air which has been heated by the hard drive and motherboard, not by the CPU. (Most GPUs vent air directly out of the case.)