For me it tends to be the heel-cup that leads to blisters without socks - I've got a pair of Nike racing flats where I even have to be careful not to wear thinner socks. I always wear socks and always make sure they're made of a technical fabric.
However, with Vibrams I never wear socks and have never had any blistering problems other than the usual bearable ones on the underside of my foot. I put it down to the low heel cup, a very soft lining and pretty much unnoticable seams.
Edited: 15/09/2012 at 00:27