This is an interesting article on over vaccination of pets. There are varied opinions and most are just that "opinions." This doctor has been studying the issue for several years.
Schultz: Dog vaccines may not be necessary (March 14, 2003)
Seems to be a pretty balanced approach. I do believe in dong titers before re-vaccinating. It is not a huge expense. Why keep exposing the immune system to these things if it is not necessary? It is the vaccine companies that make the guidelines and they do have a vested interest in frequent vaccination.