More specifically many questioned whether they should buy GBP/USD the day before brexit or the day after. The only good advice is to buy 50% the day before and 50% the day after, that way you are wrong both times.
I tried writing out some general advice, but there's so many possible scenarios: is the company hot, is growth flat, is it trying to sell to someone (PE firm or later stage investor), why are they even buying your stock?
I agree with ralph84 that selling some smaller number keeps you in the game and also let's you cash out - founders and early employees regularly do this, and they are big believers in future upside. Also remember that even if the stock goes up in the future, you should price in the cost of having that money now (everything from the appreciation of a house you may buy, to factoring in expected value).
Main advice: get a lawyer. If you're looking for someone to talk through the options informally, my email is in my profile.
The last point on interest rates is overlooked by casual investors. Interest rates drive valuation, because the risk-free Treasuries rate is the ultimate comparable to other investments. Not only does it cause the P/E of mature companies to blow up as people chase yield, it forces hot money to chase earnings growth as prospective returns get miniscule. It also gives funds massive borrowing power.
Basically, consider the risk of the whole thing blowing up even if the company does not.
The last line sounds scary, and it could be, and if you don't know it's the thing to really consider. Risk versus reward. Selling half seems like a brainless way to make a decision, but the benefit of this is that it actually lets you make a decision (you win psychologically both ways).
Companies do not do stock tenders all that often and it could be a good sign. But it also happens that companies buy during good times when cash is flush.