"Please forgive me. I'm unbelievably nervous. I'm not very good at this." ~ Katherine Heigl, presenting at last night's Academy Awards
Yep, even actors can be afraid to speak in public!
A few months ago, I posted about apologizing during a presentation, either to explain away nervousness or to get the audience on your side for some as-yet-unperceived mistake.
If you've done nothing wrong, there's no need to apologize.
If you're nervous, like Katherine Heigl, I guarantee you are ten or a hundred times more sensitive to your own nervousness than the audience. And even if they do notice you're nervous, so what? They would be nervous, too, if they were the ones in front of the room.
You may think that apologizing in advance is going to get the audience's support. You might think it will endear you to them. But really, they don't care. In fact, you've just drawn attention to the fact that you might screw up, and they may even start looking for something bad to happen.
I'll say it again: There's no need to apologize when you've done nothing to offend. Keep your insecurity to yourself.