I always brush the teeth and rinse mouth with tapwater, my understanding is that the tapwater is not contaminated or anything with just heavy with minerals & salts that will build up over time.
But, I've been in Thailand so long that my system has adjusted to here, when I go to Aust for a holiday I always go through all sorts of unpleasant things until my body re-adjusts to the water & change of diet. Last trip to Perth in April was terrible, it was great to get back to Thailand and quickly settle down to normal again.