Honest question: why? Can't your card / bank / PayPal do the conversion for you? (I say this as a Canadian who often pays USD with my Canadian card / bank account).
The bigger issue for me is pricing in USD, and less about the currency in which payment is collected. I'm in the UK, and while I can generally work out a rough exchange rate for GBP to USD and EUR in my head, I've no idea what the current AUD rate is. Making me check is a small but real barrier.