Well I don't know about all this 'must be used once a month' business, but I do know that if they're a 'rolling code' transmitter (not unlike any recent-year garage door opener), then yes, there is a *slight* chance that a transmitter that hasn't been used in a *long* period of time can become inoperable and will need to be reprogrammed to the car.
In a nutshell - here's why:
Both the fob and the receiver in the car use the same 'random' number generator to generate the 'rolling' code that's impossible to predict. I say 'random' because it's not truly random, but close enough.
When the fob sends a signal, it generates a new code for next time. Likewise, the car, when receiving a valid signal, will generate a new code, which (since they use the same generator) will be identical. Hence, they are in 'sync'. Now how does this work with multiple transmitters?
Well - the receiver in the car will accept any of the next 256 possible codes that the transmitter might generate. This way if you're 'button happy' and press "lock" a number of times while you're away from the car, it will still work even though they are no longer perfectly 'synchronized'.
However - if you've not used a particular fob to do any of the previous 256 functions (lock/unlock/trunk/panic?), then it could stop working completely and need to be manually re-sync'd. So if you typically lock or unlock your doors 256 times in a month, then yes, monthly would be required.
I can't believe that Dodge would require a visit to the dealer to add/remove a fob to a car. Some cars you just have to turn the ignition on and off like, 8 times, press a button on the transmitter(s) you wish to program to your car, and presto?!
