IMHO, jackets aren't needed this time of the year. We just got back, and never needed jackets, long sleeves, or long pants. I know that some people think some of the indoor restaurants are chilly, but I haven't found that to be the case for my family.
I would say that it never hurts to take one, unless you don't have any room left in your suitcase. I was at the CR for dinner in May and they must have had the a/c on full blast. It was freezing in the hotel and in the restaurant (CG).
DO consider bringing some insect repellent wipes for nights however - no-see-ums and mosquitos are starting to show up after dark! Really noticed them last weekend sitting on the Boardwalk (by the water) after dark.