Drexion: I'm also overly conservative in my estimated tax payments and typically my largest one is on 1/15, the fourth and final quarterly. That will be the case this year. I had sizable tax loss carryforwards from 2008 so made no estimated payments on 4/15 or 6/15, but did pay a chunk on 9/15.
However, sometimes in past years the IRS has hit me with interest and late payment penalties because my quarterly estimated payments were too small.