Jailbreaking is safe, unless you install some unnecessary junk on it, then you'll just lose memory. The basic capabilities you can do with jailbreaking is customizations and 3rd party apps. You shouldn't damage your iPod Touch during the process since you are only double-clicking a program. The program will run itself and you will reek the benefits from it.
The only downfall to jailbreaking on a 3g is the jailbreaking program is tethered, meaning if you turn off your device, you will need to run blackra1n again. So, avoid turning it off or letting your battery run out. But if you don't mind taking 30seconds (or less) out of your life to rejailbreak it, then no biggie.