So when they ask me if I want to use "my home rate" I decline to use my mastercard or visa rate (which is higher) correct?

If I use my Mastercard, I am asked if I want to use the Mastercard rate. And looking at Mastercards FX rates that they publish the next day on the internet, that is the option I choose as it is generally a better rate.

Never been asked about a home rate, but the other option was to use the local rate.

Sounds like Dynamic Currency Conversion (DCC).

Correct, you are better off declining the offer to charge in your home currency, and go with MasterCard (Visa) foreign exchange.

"My home rate"....well, that's a creative warm & fuzzy alternate name for the DCC ripoff. Seems a lot of banks sure try to put lipstick on a pig. DCC bad, very bad for the customer; DCC good, very good for the bank. And if your home country bank card also carries a foreign transaction fee you'll most likely still get hit with that charge on top of the lower DCC exchange rate...not because the home country bank had to do any money conversion but simply because it was a foreign transaction in any currency.

Also... beware the double-negative version (not sure if that got to Thailand yet).

Single negative: Do you want to choose dynamic currency conversion into your home currency? Aanswer: No.

Double negative: Are you sure you don't want to choose dynamic currency conversion in your home currency. Answer: yes.

Or... just walk to the next one which doesn't even make the absurdoffer.

eg. Aeon.

Last year, I used a few atms in the Baltics (or almost used them), and the last page was in foreign language, asking me to choose yes or no, with no idea what I was agreeing to (but pretty sure it was dcc).

... I mean, why is this even allowed. It's never a better rate. The question is, basically, would you like the usual rate or a worse rate.

Or as a double negative: are you sure you wouldn't prefer a worse rate?

If you pay with Master or Visa now in Central Festival, Tesco, Boots or in Bangkok Hospital and mant Hotels you must to take an look as well as now more and more Business owners in Thailand join the Scam. So make shure that you tell the Cashier that you want to pay ONLY in THB. The try to cheat me the other Day in Tesco North Pattaya..so i refuse to sign and they cancel the Whole Shopping Cart Items. After they scan it all piece by piece agian..... I think it is the only way to fighting back to this DCC Scam.

Btw.. here is grat Link to an US Site who Explain this new DCC Scam: Yust say no :http://thepointsguy.com/2013/07/just-say-no-to-dynamic-currency-conversion/

so the only way is

1 . say NO!! and 2. Let as much people as possible now about this scam.