Is KillTimer necessary?

I use SetTimer API and I see a lot of code like this:

case WM_DESTROY: 
    // Destroy the timer. 
    KillTimer(hwnd, IDT_TIMER); 
    PostQuitMessage(0); 
    break; 

Do I have to call KillTimer or the system will automatically free resources on the process exit? Does forgetting to call KillTimer lead to resource leaks?

I understand that if the timer is not needed it CAN be destroyed by KillTimer. But MUST it be destroyed manually?