Wages in the Dominican Republic are extremely low, especially when compared to the local cost of living. A 2014 report by a national bank showed that the average salary was around £200 while the standard family "basket" came close to £400. Besides, the salary of the lower jobs in the transport, catering, and tourism industries is below the national average. Therefore, it comes as no surprise that tips are not only welcome but essential to balance the family budget.
The increase in tourism over the last few decades brought to the Dominican Republic comparatively well-off foreigners paying often for one night's accommodation the equivalent of the local's monthly income. Americans, in particular, are used to leaving around 15% tip when they eat in restaurants back home, and accustomed the locals to expect similar gratuities.
Europeans have diverse attitudes towards tipping, many not used to tip in their countries. However, even when tax and service charges are included, as in the case of restaurants, an additional tip is customary, but an extra 10% is quite adequate.
In taxis, it is not customary to tip, but if you do it, you can rest assured that it will be appreciated.