There used to be definite rainy and dry periods. But in recent years, this has not been the case. Weather patterns aren't nearly as reliable these days as they used to be, not only in the DR, but throughout the world.
There are more likely to be occasional periods of heavy rain later in the summer when tropical waves and depressions are passing through the area. If you get rain during your May vacation, it is not likely to spoil your trip. Most likely it will be a late afternoon or evening shower.