Whether you're hooked up to a projector or just showing something to your boss on your laptop, the last thing you want is personal notifications popping up on screen. If you know when you'll be sharing your screen, you can create a separate account to keep anything embarrassing at bay.
This isn't to suggest that you have anything to hide, but in this era of push notifications and always-on web apps, the risk of something potentially awkward popping up on your screen is higher than ever. The idea is pretty obvious in hindsight, but it's one of those things that you need to think about and set up in advance, before an awkward disaster strikes.
Just create a new user account on your machine, and leave it bare-bones. That means default wallpaper, minimal files and applications, and no desktop notifications of any kind. As long as your computer is reasonably fast, you can switch over to this account in a matter of seconds.