Stutter worsens if you are nervous. I personally find letting the audience know beforehand helps a lot because you're no longer worried about hiding your stutter or limiting it during your presentation. If you don't know what effect it'll have on you, I would suggest you try it out and see if it'll make a difference. How I would look at it is that by the end of the presentation, they would know that you stutter if they were at all paying attention. So how much more of a difference would it make if they knew before the presentation rather than during or after?