Originally Posted by tide

I do this often. I enter airside in the afternoon, the day before a 6am flight. However, I have no check-in bags and I e-checkin. No issues with security/immigration.

Originally Posted by tide

Traffic at that time is bad - I don't like to be stuck in a cab. If it were me, I would take the bus to Mo Chit BTS or the newer buses A3/A4 to Lumpini/Khao San.

If I take a cab from DMK, I take it from the Family Mart on the main road. Exit departures door, head right, walk a little and you'll see it on your left. There is usually a line of taxis waiting for pickups.
You thus avoid the little mini-traffic backup trying to exit the airport.