It does make a difference, at least for me it did. Back in the mid 70's I installed a different heater box in late fall in my 68 GTS. Winter came and the heater blew out not even warm air. A friend and I were driving around freezing our asses off. He asked me why my heater didn't work, he asked if I installed the heater hoses correctly. The next day I switched the hoses around and nice hot heater