I don't have a lot of experience tracking price changes, but the ones I've seen have been up, not down. I suppose it's because the flights I typically take tend to be fairly full and as they fill up, chances are if prices were to change they wouldn't be going down. I guess it just depends on how many empty seats they're projecting the flight will have. If you wait until just a few days before your flight, sometimes there are Super Deals available on the Thai website that are excellent prices, but have the restriction that the ticket is not changeable nor refundable. I normally don't take a chance and just book my ticket once I've decided on my schedule and don't really worry or care what the price may do after that. If the trip is totally optional, then I might wait until the last minute and hope for a Super Deal.