NPCs are known to have a Defense Chance of somewhere in the region of 6-8% but to not have one whit of Resistance Chance as evidenced by the fact that Force/Tech attacks *don't* miss while melee/ranged attacks, even with a bit of accuracy, *do* miss. Blade Storm doesn't miss because it's a Force attack (100% base accuracy) and NPCs have no Resistance chance (0% chance); combine those 2 aspects and you get a 100% hit chance without every stacking a point of accuracy. Special Melee attacks miss because they are special melee attacks (explicitly listed in your character tab as having 100% base accuracy compared to the 90% base accuracy of a basic melee attack) and NPCs have a known Defense Chance (in the 6-8% range); even including the ~4% accuracy, those 2 facts combine to get you a roughly 2-4% miss chance. Keyboard Ninja's 5% miss chance number comes from 6% defense and 1% legacy accuracy. It's not some arbitrary 5% additional accuracy for melee attacks.
So just wanted to touch on this bit.
At least at the raid boss level, it is in fact 10% Defense chance.
Easy to test on the dummy, and the overall Accuracy numbers on dummy do align for the most part with actual content ("for the most part" to account for margin of error).
There was also a thread on mmo-mechanics (which I am still looking for, I'm having one of those insufficient-coffee-intake moments and my memory refuses to cooperate) which lend significant additional credence to a 10% chance instead of 6-8%.
That being said, it doesn't directly change the discussion at hand here. I don't have a Juggy/Guardian at cap or a basis of empirical data (and anecdotal evidence based on personal performance) that would lead me to suggest a particular "ideal" Accuracy value.
What I do feel comfortable saying, though, is that capping Accuracy requires significant investment in Accuracy rating, and I do not see any reason why that investment is worth making for Juggy/Guardian, knowing the cost in mitigation pool to achieve that target.