Shield Bash's stun isn't 100% but it works extremely often. In a Moogle party once I had a crappy tank/bard and ended up kiting the Warrior moogle around for half the damn fight on my WHM and shield bash never failed to stun. I'd be more worried about it missing than it not stunning.

If it's needed in order to land Goring blade there's no reason not to as the defense down is amazing.