I find that I cannot, due to mine being too hot all the time. Even under fairly light load, my temp is 60-70 C and the bottom shell gets very hot.
I can't speak to anything but my new Haswell 15" rMBP base (2.0/8GB/256GB), but mine just barely gets perceptibly warm during light use on my lap. Anything more than light use and I'm likely at my desk, but I've not felt this thing get warm at all.