The short answer is: it's a moving target. In my view, you'll do best if you show up with U.S. dollars and take your lumps when making purchases in Euros. Sometimes, you'll be able to use dollars at par with Euros (i.e. 1:1), and other times you'll pay a premium using dollars (such as at the Simply market in Hope Estate, or at restaurants that don't offer 1:1). Overall, there are better ways to spend your time on vacation that worrying about a few cents more or less on the conversion rate.