I carry whenever I legally can, and I carry with a round in the chamber (most often a 1911 in condition 1, but occasionally an H&K P7M8 with a round in the chamber). I'm not clairvoyant, so (1) I have no way of knowing how quickly I may need to put my gun to use if I need it; and (2) and I'm not willing to count on have two hands available.
This subject has been discussed a lot. See --
http://thefiringline.com/forums/showthread.php?t=361056
http://thefiringline.com/forums/showthread.php?t=368375
http://thefiringline.com/forums/showthread.php?t=399039
http://thefiringline.com/forums/showthread.php?t=389748
http://thefiringline.com/forums/showthread.php?t=398481