Don't accept instant payments on your payment gateway and always wait for confirmations. 1 confirmation is probably enough in most transactions to see if its able to be reversed or considered to be risky. I would advise that you always wait for that 6 confirmation to come in before processing the order however you could accept the transaction and get processing ready. Bitcoin is not meant to be used for instant payments and instead its to be used for high security payments and that's why its always best to wait for confirmations.