First we know there is no standard for stiff. And like Tom Wishon says the bending of a shorter iron shaft isn't that much for the average swing speed.
We don't 'offend' the shaft as much with an iron as we do with a wood. So flex isn't going to matter that much for most of us in an iron. Unless you have a high swing speed and / or a late release, then flex matters more.
The main criteria for graphite in irons is weight and feel. I have chronic elbow problems and choose graphite for those reasons.
There's a video I've posted in the past with British teaching Pro Mark Crossfield here he hits a 6 iron with a stiff, regular and ladies flex and his numbers were all very similar. He averages 260 yards with his drives and is also one who doesn't 'offend' the shaft with his swing, so shaft flex matters little for his swing.
Again, with my elbows I'll never go back to steel in a set of irons. Shot an even par 35 on the front 9 today with my weak Ping CFS graphite shafts. And like Wishon says, most average swing speed golfers could play a full flex softer and not notice a difference. My Ping CFS graphite shafts are proving that.