If you scann the emails on your edge servers before it hits the journal mailbox I think thats fine but I dont believe it is recommended to activily scan the actual journal mailbox
Everything else about EV states to avoid AV scanning of the index directories and the vault store directories so I dont see wht the Journal mailbox would be any different. My guess is avoid scanning the journal mailbox
I may be wrong...it woold not be the first time :)