darkmage - 7:43 am on May 6, 2009 (gmt 0)
leadegroot is right about the GST - depends on the who pays you - up to a point. The income would have to be split between Australian and non-Australian income (Google Australia won't collect GST on its foreign ads). This gets messy.
Out of curiosity, has Google implied how much advertising is from Australia for your sites? I think a high percentage is possibly why they are asking you about AUD.
The only benefit I see is a much smoother and predictable exchange rate if it is done daily.