You won't ruin your PW by merely jailbreaking it. There are probably hundreds of thousands of jailbroken Kindles out there, and none of them just turns into a brick because it's jailbroken...
You simply shouldn't do stuff that you don't understand on a jailbroken device (just like you shouldn't simply delete random files on your computer if you don't know what they're good for). But using the pre-packaged hacks and following the instructions, you won't harm your Kindle.
Anyway, this is probably getting off-topic for this forum. You're welcome to join the "sister thread" in the dev subforum
