I know its been a big discussion point MANY MANY times..... BUT
We found a really good money changer in Seminyak.
The guys name is Rudi and he used to work at the Pt Prawata money changer opposite Bintang supermarket.
His shop is opposite SOHO diner and he is SOOOOO honest its scary :)
A couple of times when the exchange rate had gone down, he gave us the higher rate of the day.
Give him a try, we were never more comfortable.