One reason I like to yell/scold first is that it tells all the other hounds what is happening. Generally they will all come to me if I yell, and it reinforces the no deer rule with them if they have smelled it too. If I were to just press the button without yelling, all they would hear is suddenly one of them going nuts without some of them knowing why. I find scolding serves as a warning to the others (most of whom are no longer wearing collars), and reinforces their earlier training. By the time my puppies start hunting, most have been scolded a few times around the yard for one thing or another, so they recognize displeasure from me.
My method works well for running a big pack of hounds, but everyone seems to have his/her system. With training collars, it should be very easy to break hounds from trash running. If you still have difficulty, you may need to determine if you have the right line of hounds. We used to have to run hounds down in the days before collars.