You aren't required to convert your earnings to USD at any particular point, but you are required to pay your US taxes in USD.
You are also required to report your income in USD on your US tax return, but that doesn't mean you have to exchange your currency from GBP to USD.
For example, if you earned 1000 GBP during the quarter and you want to pay 20% in estimated tax for that quarter:
1000 GBP / .74 = $1351.35
200 GBP / .74 = $270.27 tax payment for the quarter
You would need to pay this amount in USD, but your don't have to convert the rest of your earnings, unless you want to.
Also if you keep your books in GBP, you can just convert the amounts to USD when you file your tax return, using the average yearly rate.
You can use the annual average currency exchange rate, so that you don't need to keep checking the daily rates. Click here to see the published rates.
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