I recommend doing it before it starts. It will be a huge relief off of your shoulder and you don't have to worry about hiding it. Once you get the pleasantries out of the way and they start with the first question, "So tell me about yourself...", You could say something like, "Before we get started, I just wanted to let you know that I stutter. It might take me longer than usual to get some words out but please be patient with me." Or something to that effect. There's many ways to say this. Most of the time people are very nice about it. I never had anyone not respond well to it. If not, it's not a company you want to work for anyway. Good luck!