I usually negotiate a fare and avoid the meter which seems to end up costing more.
At the airport, the regulated taxi service desk is inside and has a set fare to the hotel with instructions for the driver. The only extra is if YOU want the driver to use the Expressway, which saves a lot of time and surface streets. THey will ask you, if you want to spend the extra dollar or so for toll. It is best to say Yes and pay it. This Xpressway is a major improvement over my first trips to BKK about 6-10 years ago.