It should be blowing out of the case.
not true... its system dependent.
example - i have 4 case fans in the above example. drilled 2 holes on the side pannel, 1 over cpu and 1 over gpu... i have both of those blowing in.. so i put 2 fans on the back blowing out for a steady balence.
if you only have 1 case fan total.. and if your PSU fan blows out, you should have the case fan blowing in.... if your PSU fan blows in, the case fan should blow out. I dont think anyones psu fan blows in though.