Hi there, great question.

When dealing with professional investors, especially VCs and active Angels, it's usually pointless to ask for an NDA. They'll reject your request out of hand and your request will mark you out as a newbie.

The basic reason for this is that active investors and professional investors don't want to steal your idea. They've got reputations to protect. Stealing ideas will get them blacklisted in the startup community. There's nothing more valuable to an investor than their reputation and besides, they want to invest in good ideas and teams, not steal from them.

It can be helpful to use NDAs with technical experts (assuming they'll sign it) and with vendor/service companies for your startup. There are NDAs available to entrepreneurs. They're definitely of varying quality. If you want to develop and use an NDA, I recommend hiring an attorney to help you when you're ready.

Here's a video I put together on this topic in case it's helpful: https://www.youtube.com/watch?v=yzgpJMXPGZc