Is it a good thing for my dog to bark sometimes?

My dog loves to sit on the back of the couch and look out the window. He rarely barks, but if we have company or the FedEx guy walks up to the door, he goes balistic! It annoys us yes, but we're on the fence about disciplining him...since he's alerting us to strangers. Is it okay if he barks in this manner? He doesn't bark at any other time unless we're playing and he gets too excited lol.