Bootmii in boot2 gives the best protection as it sits before the system menu.
If you install pre-loader it modifies the system menu and offers not much protection as bootmii does.
Just backup your nand. Store it in a safe place and you can experiment with this wii as much as you like. As long as you keep bootmii in the boot2. So don't put 4.2 on it as it will remove bootmii (you can install it afterwards though)