The DR is a poor country. The resorts make a lot of money and the workers make very little. They work crazy long hours and make somewhere between $100-$200 per MONTH. Many support large families.
When we go, we always try to tip everyone that we can. It is very much appreciated there. We purposely overpack & leave clothes, my fiancé takes new baseball hats and gives them out (baseball is big), we also tip cash. They appreciate US money - more value. Leave a dollar or two daily on your pillow for maid. Do daily because different maids clean your room depending on schedule. But tip the people you mentioned and bartenders. I would tip the chef but we stay at all inclusives and no guarantee it will make it to him back in the kitchen.
They are very kind people and if you tip someone a little something, he/she will cater to you for the entire trip (no lie!).