Interesting question. The discharge seacock is only used when emptying the holding tank into the sea. There are areas where it is legal to discharge and places where it is not. In my area, overboard discharge is not allowed with 3 miles of the coast line.
You are also required to provide some sort of protection that prevents accidental discharge when in no-discharge areas.There are several ways to do that: A system may have a selector valve that allows the holding tank to connect to the overboard discharge or the dock pumpout fitting. If you have that valve, it can be locked in the pumpout position. I believe it is acceptable to use a tie wrap instead of a lock ( the Y valve has holes in the handle and valve body for a lock.
If there is no Y valve, the handle of the overboard discharge seacock must be closed and either held closed with a tie wrap, lock, etc or closed and then the handle removed.
A third option for some electric toilets is a key switch that disables the macerator motor for the holding tank.
All that said, in general seacocks should only be left open when the outing needs it open. For example, when docked, all seacocks should be closed unless needed ( AC raw water for example if using AC)
The seacock for overboard discharge should only be opened during actual overboard discharge operation.