Two weeks ago, when I checked-in at San Francisco, I wasn't asked for an onward ticket, but I have been asked several times in the past. When I arrived at Phuket Airport, I was asked for an onward ticket by the Immigration Officer, which I understand is really rare. So, it's best to have an onward ticket within the 60 day visa-exempt period. Some people "rent" and onward ticket from onwardticket.com or one of their competitors. They are real tickets that are automatically cancelled a couple of days after you get it.
The other benefit of getting an extension instead of overstaying by one day is you could be stopped by the police on the way to the airport, then jailed, then deported. The odds of being stopped are very low but not zero. The consequences are very high.