Depends on state law. In most states where I've looked at gun law, it's the firearm, not the ammunition, that determines whether you are carrying concealed or open. Here in Texas you must conceal your firearm (and have a CHL of course), but you can hang the magazines from your ears if you wanted. Course that makes it real easy to presume you are concealing a gun.
I wouldn't sweat it. Put your extra mag in a pouch; if you're OCing it's an extra mag, if you're CCing it could easily be a Leatherman (or if you have a two-mag pouch it could be a PDA or cell phone). Either way, any antis you see won't make an issue of the pouch as much as they will the gun.