My boat has usually had a slip for the season and I would never leave my boat without flushing the motor. After every, and I mean every salt water adventure I at minimum hose down (usually wash) the hull, but ALWAYS run fresh water throught the motor. Even if it means breakin out the ear muffs on older motors I would do it.
One of the nice things about visiting fresh water on vacations, like Lake George, use tie up, hop off and continue to drink.

JJBroc