Yes - ALWAYS. Though the amount varies depending on what gun I am carrying and what kind of clothing.
REMEMBER: NOBODY EVER LOSES A GUNFIGHT BECAUSE THEY HAD TOO MUCH AMMUNITION!
I always have at least one extra mag (or speedloader) on me whether CCing or OCing.
There is a company called Passport that makes a lot of inexpensive holsters & pouches constructed from woven nylon. I have found that I really like their double-mag pouch. I am able to fit 2 single-stack 1911 mags in each of the pockets, for a total of 4 extra mags (4x8=32 cartridges). The pouch costs all of $14. The key to being able to carry a pouch containing 4 mags on my hip opposite my gun is [drumroll] a GOOD gunbelt. I have a Blackhawk kydex reinforced leather belt (actually, I have two - one brown and one black), which is absolutely incredible. It makes it much more comfortable to carry a gun on one hip and a mag pouch on the other. This belt does not look like a gun belt in any way, shape, or form. It looks just like a run-of-the-mill leather belt. Don't spend $80 on a belt from Comp-Tac, or Galco, or anybody else. The Blackhawk belts I have can be had for about $45 each.
The same Passport mag pouch can be used to carry my double-stack .40 mags (13 rounds each) for my Glock 23. Northerner - I see you're using the CA-approved 10-round mags. Why not pick up the 13 round mags?
OR, it can hold 2 SR9 mags (17 rounds each).
Sometimes, regardless of which automatic I am carrying, if my clothing permits, I will slip an additional mag in a pocket, whether I am carrying the 9mm, .40, or .45.
If I'm in "deep cover" mode for the day, carrying just the LCR, then you can bet your backside I've got at least one, if not two speedloaders squirreled away somewhere. I tried the Bianchi speedstrips, but even with practice I am not nearly as proficient at quickly reloading with those as I am with the speedloaders. Though they are much easier to carry than speedloaders, I either end up taking too much time, or I drop one of the cartridges somewhere along the way with the strips.
With extra mags, I always figure that just like the gun itself, better to have it and not need it than to need it and not have it. If it is feasible to carry "just one more" mag - do it. I'd hate for my last thought before taking one between the eyes, execution style, to be "Damn, I should have brought more ammo."
EDIT: Its late, and I'm going to bed, but tomorrow I will try to post some pics/links for the stuff I'm talking about.