I'm looking at making a quillon dagger to the specifications of the ABS Master Smith certification (which brings up a whole can of worms that I don't want to get into on this post).  I was looking at pictures of WIP online and noticed that a few of them had a step down kind of thing going on with the tang.  Is there a reason for this?

I've been making knives for years and haven't seen this technique used.  Maybe I'm just living under a rock!

I don't do mine that way... but I can see why you might.  Say that you want a through tang that has a small rivet at the hilt end... yet you desire a strong, robust tang where the blade meets the handle.  That would be one reason for a step down tang.  Usually I would just use a partial tang instead... possibly tapered a little.