If you mean, "Hacking your Wii", as installing the HomeBrew Channel, then yes, it's fully safe.
But if you mean, "Hacking your Wii", as installing a custom IOS and changing the Trucha Bug to IOS36 and stuff, then no, unless you have BootMii as boot2 installed and have a backed-up NAND.bin file.
----------
Also, never update your Wii because these updates are just tactics to prevent Wii consoles from being hacked.
Proof can be found here: http://wehackwii.pbworks.com/Firmware
Even though there are ways to hack your Wii in the 4.2 update, you'll need BootMii as boot2. Because if you have 4.2, BootMii only as an IOS will be useless.