You'll generally have a little forward press with your irons if your ball position is in the correct location. Generally most people play the ball to far forward with their irons (like a driver position) thus the hands are behind the ball which in turn causes fat, thin and topped shots.
If your ball position is correct, you really shouldn't need to consciously think about forward pressing. If your ball position is incorrect and you're trying to forward press simply so your hands are in front of the ball you'll likely cause more problems for yourself (although you may be able to get away with it for a few days if your timing is on).
Here's a video of Ernie:
YouTube - Ernie Els Swing Vision
Notice his ball position and position of his hands. He's not really forward pressing so to speak, but it looks like he simply because of his ball position.
That's my .02