San Juan taxis have fixed zones; beyond San Juan, you can get a fix-rate car or van service to major resort areas east of San Juan (or to Fajardo if you are catching the ferry for Vieques or Culebra).
Renting a Car: It's not really worthwhile to rent a car if you are staying in San Juan; taxis are more cost-effective. However, if you want to explore the island for a day or two, then a car is a necessity. A car is also needed if you are staying somewhere else on the island, with the possible exception of a large resort that you don't plan to leave except on a guided tour (though even some of those charge for parking). A car is almost a necessity if you are staying on Vieques or Culebra and want to explore the island or visit a distant beach.