To me, it's a personal decision and the responses here bear that out.
With my semi-autos (ususally a 1911) I don't carry any spare ammo. For some reason, with my 5-shot revolvers I often carry a speed strip. It's not that I feel under-gunned with the revolvers, but probably because the speed strip is much smaller than a spare mag and I can just toss it in my pocket and not even know it's there.
But as many have already stated - it's really a matter for you to decide, based on your own comfort level.