kylieh, for the $20-$30 that you are going to pay, if you are concerned then just get one. But as I have posted earlier, in 18 years of fairly continual travel to the USA, from my personal point of view, I have NEVER EVER been asked for one, from the booking of the rental car to being licence checked by the "Chips".
Getting an International Drivers Licence for the states ia a bigger con than climate change.....that'll get me in trouble with the left!!!