I personally make it a point to carry at least one spare mag, often two. At least one for malfunction clearance as you mentioned, and also because sometimes, even when the gun is working perfectly, we are shooting less than perfectly or the situation otherwise demands more than what is in the gun to start.
Location? It's personal based on factors of comfort, habituation, and dress code. I've never been comfortable carrying spare mags in the pocket, except in the "cell phone/magazine pockets" of some tactical pants. Usually I'll have the mag(s) in a pouch on the opposite hip from the gun, outside or inside the waistband depending on the degree of concealment needed.