negative or positive thru relay switch

I have a circuit that works but would like guidance. I searched the forums but didn't see a reference to my particular question. A doorbell switch and transformer makes a 30amp relay close. The NEG side of a spare 12v battery connected to an air horn is switched by the relay, activating the horn. It is for the garage, of course. and works great. Does it matter if the NEG or POS is switched through the relay and why? It seems to work either way. Also, is protecting the relay with a diode unnecessary? Thanks

The diode is necessary if it is making a pop in your audio system. The relay is just like a switch. It does not matter which wire it is on.