Ultimately it is still legal and ultimately it’s up to the immigration officer dealing with your passport. And as long as I’ve been living here, over a decade, that’s the way it’s always been. Now that they have the DTV immigration may well be getting more stringent I. Order to sell more DTVs. But who knows…
Just make your reservation for the return trip home from Thailand when you know when you’re going to leave. Immigration rarely asks for a return ticket and if they do just tell them you are traveling out of the country by land.