When I was there in Feb/Mar I topped up my old rabbit card a couple of times at Sala Daeng and was not asked to register. I think the little display even said something about it not being registered. The first time was after a small group of people buying new cards took a long time to get things sorted out and there was a big line. But the second time there was no line. So I thought maybe it was just a matter of luck of the draw.