Somebody (Jim?) correct me if I'm wrong, but the centre of percussion (sweetspot) on most irons is slightly towards the heel, while on woods the sweetspot seems to be more in the middle. I bought a set of Mizuno blades off a buddy of mine a long time ago who is/was a great player (mini tours) and the wear marks were towards the heel a little.
The first question should really be, is there a problem with your contact pattern? Are you $hanking your irons at all? If not, you may be hitting them right where you want them.