Yes it is wise to use a check valve. You will be better off putting a check valve on each line exiting the gang valve. Between the gang valve & the tank. I found out the hard way that not all gang vlaves are water tight. During a blackout I ended up with a flooded cabinet because heaps of water flowed out of the gang valve when the check valve was between the airpump & the gang valve.
I'm sorry that you seem to be upset by my response Cichlidude but I'm not the only person I know that has had the flood happen froma gang valve. If you just want to run one check valve between the air pump & gang valve that's your choice.
My gang valves are the green Pen-Plax ones Jrobber.
I'm sorry, but the check valve is just in case water back siphons to damage the pump. I see that you are assuming that the gang valve is defective itself therefore having one check valve per output. I've never had that... but I guess some have.
I'm not half as concerned about water going back to the airpump as I am having all the stuff in my cabinet (powerboards etc) & the carpet around the tank soaked. The trouble is you don't know if your gang valve is defective until you have to clean up the mess. You could go to the effort of dumping the gang valve into a bucket of water while it's all hooked up & with the air pump running to see if it leaks, but why bother when check valves are only like $3 each.