There is zero point to going 12-12 with autos; if you're going to do that, you might as well just use conventional 12-12 photoperiod strains.
The whole point of autos is that they'll flower under more hours of light.
That way, as one example, with 5 weeks of flowering under 20-4, you could "cram" in the same total hours of flowering light as 8 weeks of conventional 12-12. More light during flowering = bigger plants.
Personally, I'd go 20-4 since I think a little bit of 'dark' time probably helps.
The only reason I'd go less than 20-4 with autos was if overhead space/height were a limiting factor, in which case, less light should mean slightly shorter plants (though with less yield).