I carry a spare mag for my 1911, but not for my M&P9. 7 rounds is a little low for my comfort, but I'm ok with 16.
I really want to try and find an IWB holster for my 1911 that has a mag pouch right there with it. Maybe one of those hybrid holsters could be made that way.
I carry two extra rounds in my mag carrier (Galco Shoulder Holster) and keep extra ammo (and an extra gun) in a chain down safe under the seat in my truck.