Changing money is no hassle on Koh Samui as I mentioned in my tourist trap tip.
This general tip is about the f/x rate to the Baht. I know people who while on holidays stress about the f/x rate dropping. I try & change money every couple of days, sometimes you win this way , sometimes you lose, I personally don`t worry about this.
This is how I rationalise it using $AUD to the THB. If the rate today is 27 THB to the AUD, tomorrow it`s 26.8, change $100AUD at those rates -2700THB- 2680THB.The difference is 20baht which is only a songtaew ride down the main road,even if the rate went from 27 to 26 x 100 it`s only 100 baht not worth stressing over