How much damage could one do to an unrooted device?
From within the Android system itself, all you risk on the great majority of devices is losing data such as media, apps, logs and configurations. Anyhow, if one manages to damage it more than that, it is usually repairable through a factory reset.

Whether or not your device is rooted, you can always do it harm by flashing bad bootloaders, recoveries/recovery images, inadequate update.zip files and a few other things, so always watch out for that.

I strongly recommend that you do a backup anyway -- it's a fairly simple process and it's worth it just to stay on the safe side.