Hard to say if it's "best" to rent a car for the entire week - it all depends on how often you plan on leaving the resort.

For me, I always rent a car when I travel and I don't mind "paying" even though the vehicle might sit in a parking lot all day. I like the idea of coming/going whenever I please.

For what it is worth, taxi fares can mount up quickly. Two round trips a day to almost anywhere on the island in a taxi will cost you more than a rental car and in many instances one round trip to some of the more distant places will exceed the cost of a rental car. You can rent a car for around $45US to $50US per day and a minimum one-way taxi fare is going to run you $10US to $15US - longer rides will be in the $25US to $40US range. YOU MAKE THE CALL.