(raises hand) I take the lane when I think it's safer.
I'm a newbie to commuting, I've been riding daily for almost a month. This is something that didn't take very long to learn in this city. If there's no bike lane or shoulder, then it's safer to take the lane.

When I crowd myself over to the side, the motorists do not allow room, they simply brush me off the road. Technically, our state law and municiple code says to keep right as far as practible. For me, that's taking the center of the lane. Only when I take the center do the cars pass me giving the state required 3'. Only twice has a motorist tried to crowd me out while in the center of the lane. It was happening routinely when I rode the white line.
EXAMPLE (photo taken from Google maps street view)

link to larger picture
http://home.comcast.net/~eddyseals/pix/road.jpg

typical traffic, busier during the peak, 5pm-6pm window. The speed limit is 40mph. The cars typically travel faster than the posted speed.