Temperture setting?

[Such5]

I was wondering what others have experienced with initial temp settings. It seems like a few different times I've done work on or shut down my kegerator for a while, and went to start it back up, I've have to re dial in the perfect setting. I guess maybe I'll explain exactly what I'm getting at. I have started the kegerator up with no keg inside. Then dialed in the temp. to a consistant setting of 38* liquid temp. This was done with a large glass of water placed on something to get it to the middle of the kegerator, with a calibrated thermometer. Then when I inserted the keg, my temps changed. I thought initially that it was just taking some time for the keg to get to 38*. But I have let it run a few days before having to changed the setting in order to get the temp back to 38*. I guess I'm wondering if there is a genal rule of thumb for the setting. i.e. with out the keg (basiaclly an empty kegertor) Maybe a liquid temp of say 34* or something?