If you must change bring dollars as the have the best exchange rate, but personally I like the ATM route as I have low cost and a good rate at my bank.. but do check this prior to leaving..
The EUR/RDP spread is only 1PIP, (PESO) so it is not really that disadvantageous for someone here for a week or less to use EUR to buy RDP. It was much worse previously. If you can tighten the spread between USD/EUR it's worth bringing USD, otherwise on $1000 you save a whopping $US29!
over the last weekend I was offered 33.5 on the streets of sosua, but they bumbed it up to 34 when i tolf them I was getting over 34 in santiago!. In santiago at two different places i got 34.25 and 34.50. Plus the casino gave me 34.25 as well. Bob