Paying for things
No matter how short your stay in the US, at some point you're going to have to pay for something, and there are three ways of doing it ... cash, credit card or traveller's cheque. Don't try to use a UK bank debit card. While it's possible that you may be able to use it, they're not so readily accepted and can create a lot of confusion.
And don't even think of trying to pay with a British cheque (although there are schemes with some charge cards that may allow you to cash cheques using it).
Of course, the exchange rate matters, and if it is particularly good you may want to buy your cash and traveller's cheques well ahead of your trip. Remember that any credit card purchases will be charged at the rate when the transaction is processed. Regular travellers might keep an eye on the exchange rate while they're away and use their cash or traveller's cheques if the exchange rate has dropped below what they paid, and use their credit card if it has gone above what they paid for their currency.
Transactions in dollars will nearly always attract a commission or service charge, either as a fixed fee, a percentage, or just as a poorer exchange rate. The exchange rate that you see quoted (as on my dollar pages) is sometimes the "interbank" rate - the tourist rate is anything from 3 to 6 cents worse. But don't worry about tiny fluctuations in exchange rates - it really isn't going to make that much difference. A 1c difference in exchange rate will mean a difference of £10 on a $1000 purchase.