I have probably taken about 100 taxi’s now in Bangkok and I say with certainty that the majority of taxis in this town are good drivers and seemingly honest people.
You will have no problem with your taxi if your bell hop calls one for you from your 5, 4 or even 3 star hotel. But.. and this can be a big but, there are a few bad taxi drivers that you need to be very wary of. If you flag a taxi coming home late at night, you should definitely keep your wits about you for your own safety. The few bad apples who are driving taxis will not only NOT take you to where you want to go but they could lead you to a booby trap.
A word of caution, always have your smart phone with you, if its night time, take a picture of the taxi driver and take a picture of the taxi number or taxi license. If the taxi driver sees you do this, perhaps so much the better. Its not losing 100 Baht on a taxi ride that you need be concerned about; we are talking about personal safety here. A harmless taxi joy ride can happen any time, day or night and these rides are a nuisance because it means navigating more Bangkok traffic than you ever wanted too.
There are many taxis who will not take the quickest route from A to B just to put an extra 30 Baht in their pocket and as a naive tourist you have no way of knowing and that’s not so bad unless he really goes out of this way which can definitely happen. Stay alert,don’t assume anything, keep your eyes open, keep your smart phone handy and a little planning or help from a friendly Thai can keep you on course.