It's more or less your actions that will brick your phone... IE: if you unplug it while it's between operating software's, if you unplug it when it says not too, if you pull the battery thinking it froze when in reality it's doing some loading. Downloading incorrect files, unzipping files not meant to be unzipped, placing files in in correct folders, or removing or tampering with important files... Just to name a couple. If your already rooted then you should be fine.