This advice is especially true of "big" companies trying to blind you with what you think of as "huge" potential only to try to see who your top employees are and whether or not they can poach them.
People who are serious about a potential acquisition will tell you, without any other information, how they would price such a deal. Don't start due diligence and the distraction it entails until you have both a signed memorandum of understanding, and the potential acquired has to pay you if they back out of the deal.
How is this done with the lots of sub $10M acquisitions that happen? Clearly deals are being done here, so I'm sure it doesn't pay to ignore bona fide offers, but obviously there's a lot of time wasters out there too.
What do you advise your YC portfolio companies to do here? Just ignore all offers or refer them to their lawyers or investors?
This model of the world doesn't imply any specific strategy you should follow, but you'll find that adopting it will change the way you think about the situation. You'll stop bending over backwards (why bother, since nothing is going to happen?) which will in turn make them take on the burden of figuring out how to make the deal happen, which they'll do if they're serious.