The purpose of the breathing is to help you get tight and stay tight while lifting the weight - any order of things you do that accomplishes that end is good.
@Pavel Macek, I practice a "set of singles" in order hone my competition form, but there are some interesting things about touch and go. One is owning the weight - a weight you can use for a light touch-and-go, meaning you only just graze the ground with the plates before coming back up again, is a weight you own. I like 5's with 75% or so for this.
Another interesting thing about touch-and-go sets is that your starting point for every rep after the first will sometime be different than the first - the touch-and-go position is usually a better one, and thus worth finding and trying to get into at the start of your first rep.
Touch and go sets are also a way to give a hypertrophy boost to the deadlift, since the lift then has both phases. I am using mixing them now into my training, but when the meet gets closer, I will switch to doing almost all sets of singles.
-S-