My experience is that it seems the Thai Customs officials in Bangkok tend to focus on passengers with (for lack of a better term) non-standard baggage. Boxes, shopping bags and the like appear to be a preferred target. They also seem to have become a bit more aggressive in general about inspecting arriving passengers since the move to Suvarnabhumi but that said, it's still a pretty laid-back operation overall.

As an aside, Thai Customs for arrivals at Chiang Mai has become rather aggressive in recent years. I haven't been charged duty at CNX on anything I have imported, but I have been inspected more often than not and have observed that I wasn't being singled out for inspection.