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.