The main thing you want to avoid in a non-tourist area (ie Santiego) is shorts. Look around, and you will almost never see Dominicanos wearing them, no matter how hot it is, even in PP or Sosua. In addition, they are not permitted at a number of the historical sites, and in many clubs or discos. Either dress pants or nice dockers style with a tasteful collared shirt and dress shoes is typical evening wear at all but the most exclusive places. If you have a nice tropical blend suit or 2 to bring, you will not be overdressed in the evenings, but it's not necessary.