It seems like the pressure reading seems to go from ~400 to 0 when I'm not looking, but some of the regulators have a red area, marking when pressure is low. I ask because I had to order a new regulator. My old regulator doesn't really work now (none of the gauges are accurate) and my suspicion is that when the tank runs out, beer is forced back up into the regulator from the connected kegs. I'm wondering if that's because I let the tank run completely out first.
When the tank runs out the residual pressure in the tank and in the keg should be the same and will remain the same if no beer is dispensed.
Furthermore, if some of the beer has been drank the gas dip tube in a keg is too short for liquid to get up there. I don't think that beer going up the gas line is your problem. Unless you store the keg on its side or something. Its just not physically possible, regardless of keg pressure or tank pressure.