if you already had the dealership replace the thermostat housing, you might have already had the coolant flushed. (or at least paid for it).
i never pay for someone else to do things i cant physically see when they are done (who am i kidding i dont pay anyone to work on my cars).. but i never know if they get all the old fluid out and run it as clean as i would for a brake fluid flush or coolant flush.. even transmission fluid changes (which aren't you supposed to do every 3k? :P) because you know they are doing it as well as would be beneficial. Just like alignments i have never gotten one i was happy with..