You can 6A without IB, But I ran through of list of situations where you would win and would lose: I'm testing this on his Hop dash Agito and higher timed Agito.
Since Agito has little to no recovery (this move :/):
If you don't IB and 6A immediately and he mistimes a J.D you will win, you will lose if opp. knows you'll delay. Same situation occurs when you IB but if you 6A immediately and he does as well you will lose. I'm guessing it has to do with the blockstun on IB and the recovery on Agito. This also happens with ID. (Hop dash)
Doing this on a higher height of Agito it's almost the same.
5A mash loses to J.D as well and also if they Tsubaki but 5A beats his J.A, j.B.
NOW on a higher height of Agito 5A will get bodied by all those normal but you won't get sucked in to J.D because of the nerf (yay).
5A, 6A, and ID are the go to for this but it becomes really "50/50", also note if they do hop dash Agito midscreen they will end up behind you (it's a little weird though).
Again I still would just block and get some meter, his pressure isn't amazing when you anticipate what he does. But if you want to do this then go ahead yo.
Sorry this is such a shitty and lazy write up. Gomen ;-;
EDIT: Done by Cross.