I never bought tickets POS BKK, but I did a lot POS China to SFO. Fares do vary depending of origin and dates of travel, but do not usually vary that much with when you purchase. It may vary more if there are promotions.

I don't usually wait for promotions as I saw lower fares disappearing before. For example, I bought an I fare PEK-SFO RT via HKG for my May travel. I procrastinated for two days and the fare went up from ￥13K to ￥26K (?) on my preferred travel dates. I was forced to choose another flight and the fare became ￥15K.