When you feel that magnetic interfernces will put the compass off, you lock it after a good reading and make sure that you don't shift the device sideways any more. Also, if your compass works better when held flat, you lock it and then lift the phone up.
Awesome, this answers the question I had. I checked 30W a few times, and sometimes the app showed it sitting on top of a pine tree near me (like a star on a Christmas tree), sometimes it was east of the tree. I'll also be sure to take the big fan out of the window just for paranoia's sake.