John, just to add a bit to what "arjay" said:
According to XE.Com, they claim to use "mid-market" rates - details can be found here:
In my own charts I use data which takes the average of the closing TT buy and sell rates in Thailand each trading day.
IMHO it is always better to check the Forex rates from the domestic Thai banks and, as has been often said on the forum, to exchange funds in Thailand (not only do you get the onshore rate, you get a much better deal as forex spreads are much tighter in Thailand - generally western banks use unethically large spreads on their forex deals to make colossal profits at the expense of their customers)
Pretty well all the Thai domestic banks quote their latest forex exchange rates on the internet.