I've never been checked but will always have the cash on me now just in case - it's preferable to the risk (however small or unlikely) of getting turned around at Immigration or getting held in the detention center overnight.
I used a fast track service coming into DMK last week. THB1200, no queues and no questions. Plus if you want they'll push your trolley / take you to your car / taxi if you want but I just let them go after Immigration.