I've always wondered about this, and years back there was talk of out-of-position players suffering an Awareness/Play Recognition penalty. But with so many comp players running CBs at Safety and vice-versa, I don't really believe that there is any significant issue.
Also, if you run one offensive lineman out of position, remember that the WORST Awareness rating on your o-line is replaced with the Awareness rating of your Center. For that reason, I tend to run the highest rated Center in the game on my team. Right now I'm rolling with the 93 OVR Ryan Kelly, who has 91 Awareness.