I have also found that maps in themselves aren't all that useful when dealing with BKK taxi drivers, although I will print out one if a hotel's website has it. Having the phone number available and directions written in Thai are the most valuable, and knowing a nearby landmark can be very useful. When I first stayed at the Karnmanee Palace in Saphan Kwai, it was a brand new hotel and no driver knew where it was. But when I would mention Thanon Pradiphat or say "Glai rong rhairm Mido" ("near the Mido Hotel") they knew where to go.