Upvote:4
Thai immigration rules are not clearly defined or properly published and often in flux. There's plenty of speculation on the internet, but you won't be able to get a clear and reliable answer on this, even if you personally ask an immigration official. Similarly, embassy/consulate staff would be next-to-useless, since they're under a different ministry than immigration at the borders, and can't even access the immigration database. What is clear is that over the last several years, Thai Immigration has taken a bit of a dislike of frequent visitors.
I would keep getting the tourist visa prior to any future trip -- it's a long way to risk a rejection at the airport. Note that in addition to applying in person, you can also get a tourist visa for Thailand by mail at any consulate.