I won't take the lane on a 40mph or more road. The closing speed between what I can ride and what the cars are coming up on me at is just too much for me to feel comfortable.
In that 25-35 mph speed limit area where I can sustain 20mph, I don't think the closing speed is dangerous for me. I have areas like that on my commute and I take the lane as needed. I also have a mirror, though, and I do watch my behind.