Does it matter which finger you use? I'm sorry, I had to say that.
I do the same thing though, I keep a 5 gallon jug of salt water arating all the time, I dont keep a heater in it at all. when my tank needs a water change I use the water from the 5 gallon jug. If I am topping off I use fresh RO water that has been sitting in a 5 gallon jug without a powerhead. I've never had a problem.