Can someone explain the "End Patrol" function?

[Naga]

When I finish a patrol, I hit escape and click "End patrol," or whatever the button is called, while still out at sea. Is this the "proper" way to end a patrol, or should I take my U-boat right back to the dock? The reason I ask is because it allows me to use almost all of my fuel without worrying about saving enough to get back to base.