I have never accepted USD in payment. I do sometimes get customers pay me in USD by mistake after they do their own conversion, but this is promptly refunded. There have been the odd occasion where i they have done this and i accepted it, as i had something to pay for in USD myself.
I always ask that payments are made in GBP, otherwise i have to pay further fee's to change it back to GBP as commtrd says below.
When you make payment through PayPal in GBP, it should tell you what it will be costing you in USD before you submit the payment.
Edited by Toxic Cables - 7/28/13 at 10:08am