I suggest paying in pesos. When you pay in USD you incur the risk of the merchant providing a poor rate. While this may not be an issue once or twice, it can become a nuisance if you live here. As someone already wrote, withdraw from an ATM. You tend to get a very good rate. Also, I used the casa de cambio on the Conde for years, and always got a very favorable exchange. However, the ATMs simply work better.Thanks so much.
By the way, how accepted is USD in Santo Domingo? I'm assuming the bodegas wouldn't take it, but my banker back home (a Dominican woman) said they take it everywhere and even prefer it, although I think she thought I was going to a resort.