On a lower count mag like my 1911 I always use the plus one when I carry. with the 17 rnd mag in my glock I'm not so worried. But of course, always one in the chamber.
As I interpreted what the title of the thread said I voted no, but Im an idiot so whatever. I am most usually always in locked and loaded mode when carrying or even if it is sitting in HD, but I usually will not add another round past what is in the mag. I carry and shoot an XD sub 9mm and with a minimum of 2 mags on me I feel that is sufficient. My mini pocket guns though, like the Kel Tec PF-9, if I am out practicing then I might add the +1 even with the 8 round mags.