Personally, I've found that auto voltages are the equivalent of the best possible offset voltage, but many swear that auto uses more than is necessary. It probably depends on your mobo.
For me, my i7 2600k runs at 4.5ghz 1.312 (rarely it wants 1.32) at 100% load. Ditto for offset voltage; it ran identically in offset despite hours of tinkering with voltage and llc settings and running memtest to check for memory stability. Idle voltage was also nearly identical. Manual voltage was ridiculous, wanting 1.36v to be stable.
Of course, YMMV.