Arnold is an unbiased brute force pathtracer and 'AO' is resolved as part of the render. In other(older) offline and non brute force raytracing hacks like light cache and irradiance mapping are used to add the AO effect. Of course this doesn't drop you from rendering an AO AOV pass and comping in post. Arnold is actually…
If it's possible in Arnold, I would render AO as a separate pass and add it in your compositor, that way you can adjust the strength or decide to not use it at all, without rendering the whole thing again.
What are you rendering? Lowpolies? If you are rendering a HP still, it's an artistic or practical decision on your part, as Arnold being a full blown renderer technically doesn't need any AO at all.