If you don't do any sort of encryption, then when a new instance mounts the drives, it would be able to see whatever was written to the drive by the last instance used it.
I would absolutely run / without a journal if / is on media that I won't be able to access after a shutdown (a ramdisk for example)
I don't remember seeing anything in the AWS management console that would let you reboot an instance, are you talking about rebooting it from inside the instance? If you can do that you don't need a journal because you can still do a clean shutdown. I don't consider the system to have crashed. I count a crash as being when the system stops without being able to do any cleanup (kernel hang or power off on traditional hardware)