My impression was just the opposite. If you go back to Encounter at Farpoint, Worf sounds much like everyone else. Seemed to me he didn't really get to being deep-voiced yet.
You could be right, but to me it sounded more like as the seasons went on, Dorn got more relaxed with his voice.
I have two examples here that I just found to illustrate the difference I notice:
http://www.youtube.com/watch?v=0G-o2TuP_Kc#t=22
versus
http://www.youtube.com/watch?v=51CTpvCKMZ8#t=107