When using a Silverlight-enabled WCF service, where is the best place to instantiate the service and to call the CloseAsync() method?

Should you say, instantiate an instance each time you need to make a call to the service, or is it better to just instantiate an instance as a variable of the UserControl that will be making the calls?

Then, where is it better to call the CloseAsync method? Should you call it in each of the "someServiceCall_completed" event methods? Or, if created as a variable of the UserControl class, is there a single place to call it? Like a Dispose method, or something equivalent for the UserControl class.

Thanks,

Jeff