That's definitely what I have found. I'm lucky enough to have a MUP for the majority of my commute, and there's never enough traffic on it to pack the snow down to ice. It gets pretty rutted and busted up, but not very slippery - I'm still on 700x28 Conti tourers after a solid week of snow on the ground and they do fine.

On the other hand, two blocks of my commute are on polished ice, where I just take it easy and put a foot down if I have to. After awhile you get used to it, though I'd think about studs if I had to spend more time on ice.