I have an armageddon and gandiva and also 5/5 Last Stand merited, i would say stick with 12str and 12agi.
Unless you do alot of stuff outside abyssea with a gandiva and love the bow to death stick with 12str 12agi for the time being. As long as you have 12str merited the other 12 points arent going to really break you if you go 12agi, 6dex 6agi or 12dex