Do what's comfortable for you - socks or no socks with Uggs. It's just personal preference.
I wear my Uggs without socks, but I just bought a new pair of short Ultra Ugg chocolate boots and for some reason they stretched out so much that I think I should wear socks with this pair of Uggs. The funny thing is that I have the short Ultra in taupe in the exact same size and those boots are not flopping around as much as my new pair. I think the sheepskin used in my new pair of boots is not as dense as the sheepskin in the taupe pair of boots. It's kind of annoying, so I think I might start wearing socks with the new pair of Uggs.
In the Ultras, I wear a size 9 but I also tried on the size 8 and my toes were really jammed up against the inside, so I stuck with the size 9's. But in my Classic Short Uggs, I wear a size 8 - 9's would be too big in Classic for me.
But, you really have to try on several pairs of the same size Uggs as each boot (depending on the sheepskin density) is different.