Personally, with the kind of battery you have in the maxx, do you really need to worry about that? I used to on this phone, but with the extended battery, I've found that the benefit from toggling on and off isn't that much.
Then again, I have wifi at work too, so most of the time off wifi is driving between work and home.
Anyway, if it is that important to you, I second llama, as it can perform actions based on cell towers. Personally, though, I removed it as I didn't find any real battery savings from having it running.
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