Yes, closing the seacocks when hauled is a good idea for the reasons you point out. I choose to keep all seacocks closed unless the system is in use. When we are cruising, the main engine seacock stays open, however when we get into a slip for a week or longer, it gets closed to. That helps kill off any critters trying to grow in the sea strainer. When I close the main seacock I put the engine start key on the seacock handle. Don't want to start the engine with the seacock closed. You don't need to know how I learned that lesson nor how many times I learned that lesson before I came up with this idea.
We have a condition similar to Steve. Our generator impeller would only last 30-50 hours. It seems that when we are underway, there is a vacuum generated which drains the generator intake hose, so every start was a dry impeller. If I close that seacock after each use (2x day) the impeller will not fail and will last until I net decide to change it, about 200-300 hours.