I learned to fly in the 300 at Bristows Academy and CFI'd in it at Rotors of the Rockies. The standard procedure at both of these schools was fuel boost ON before engine start and ON for the entire duration of the flight. The fuel boost was switched OFF after landing to check the engine-driven pump hadn't gone TU during the flight.
The reason - as outlined by everyone else - if you leave the fuel boost on then the engine will keep running if the engine-driven pump fails.