Presently it seems you need to choose a default rate eg A$1 = 0.75USD
Currency rates change every day. I think the default currency rate should be the last currency rate that is used (eg like the purchase price of an item) At one time a couple of years ago the A$1 to US$ was 1.00 = 1.10
hence when you go and make a purchase the US$ 1.10 would come up and need to change it. If the last currency rate was used you may not have to change it
On the purchase Register in the Amount Due column, by using the last currency rate it would reflect the more actual value of A$ required at that time
"Multi Currency - default list, purchases and Purchase register"