Most of the time at BKK, I see the customs officers either stopping every nth passenger for x-ray screening or stopping passengers just so often enough that the officers have something (not too terribly much, mind you) to do. Having a lot of baggage does seem to increase the likelihood of an x-ray. Looking like a stoned hippie with a backback also seems to increase your chances of being stopped. (At last check, Thailand and many other countries in the region had laws on the books making it perfectly legal to deny entry to hippies or hippie-looking people. Not kidding.)

I've have my bags x-rayed once or twice when coming in with a lot of baggage. Same has happened in Singapore. No big deal.

I think Thai customs is more concerned with exports. There is no formal export screening for all pax, but I think a lot of behind-the-scenes stuff goes on. Every once in a while you hear about some guy with a suitcase full of alligators and a carry-on tote full of spiders who got flagged at BKK and arrested at HKG. Again, not kidding.