When you say "set the phone up for use in the DR," I presume that you mean unlocked the phone. My US carriers would do this for me upon asking, then you can purchase the SIM card wherever you travel.
I think what he means is roaming plan that include Data in minutes overseas. will depends on what you need.
Usually the free roaming they give you very slow speed in 2G, so if you have an unlocked cellphone better buy a sim card here prepaid and add data and minutes if you wants make calls, about calls maybe you dont need it because now every body is using whatsapp for calls.
Just make sure that your phone use the same bands that the Dominican local carriers use.
Claro - 3G 850 Band / LTE Band 4 -1700/2100
VIVA - 3G 1900 Band / LTE Band 4 -1700/2100
Altice -3G 1900 Band / LTE Band 4 -1700/2100