Yes, the agent will try and reattempt the transfer if the connection is dropped. This is one of the benefits of using the agent, you let it worry about reattempting and network conditions so you don't have to. The API does all you a level of control over usage of cellular data via the TransferPreferences
property. You could set this if you're concerned. Alternatively, let the user set their own preferences about data usage via the built in settings on the phone.
There is more information at http://msdn.microsoft.com/en-us/library/windowsphone/develop/hh202955(v=vs.105).aspx#BKMK_TimelinessofCompletion