due to close quarters and the close proximity of your bulbs, you can have a pretty decent difference in temp between directly at the tip of a specific plant and mid height on the walls. i would definitely check temps at the same height of your pitchers directly under each bulb. once you have a high/low temp differential in the different locations you can better decipher what's going on directly at the plants by your reading on the wall. for example... say there's an 14*F difference between where your meter is stationed and directly on the plant... and you have a significantly hot week or something... you'll know without a doubt to move that particular plant for that week. all things staying pretty equal, that temp differential should be pretty steady as well.
Well, I've done some experimenting, and at the level of the top of the pots, the temperature stays pretty stable throughout the tank. I'm still trying to figure out how to get the readings at the top of the tall plant.