Yeah, I'm pretty sure Bitt is right.

The place where your tax REALLY gets worked out is on the 1040 form in April. The withholding is just there as a convenience. You can tell them to withhold as much or as little as you like.

Although I also heard that if your withholding was too much different from your actual tax amount, there was a penalty, I'm not sure how different it would have had to be to incur the penalty.