On the positive, it's generally good to raise money when you don't need it. You're more likely to get the most favorable terms. (Or, stated diffrently, it's hardest to raise money when you need it most, with a zero cash date looming).
On the negative, it creates a high bar to clear for the next round of funding. If you don't raise enough, and need more funding later, doing a down round is extremely painful. Worse, if you have anti-dilution and other provisions, the cap table gets blown up. Lots of agita.
Public companies: different circumstances (market cap is a currency for acquisitions).
Finally, I'm respectful of audacious and aggressive entrepreneurs, but I've got a very sensitive hubris detector. A billion is an arbitrary ego number, and "We don’t entertain offers.... . . and it’s literally not worth the time of day, talking" is crazy. Tell that to Groupon shareholders that held stock during Google's (reported) ~$6b offer in 2010. Ouch. Pride goeth before the fall.