could depends on the load of the flight, if the Y class is full, then they can get strict about the carry on, but typically, no, they are not strict about it.
JFK weighs every single thing for every single airline, even after check in, you may be stop before security and weigh your hand carry! saw it last december as 1 couple had several big bags and were asked to recheck in their luggage. the agent refused to let them proceed beyond the security till they have checked in the bag. i think they are on some american airlines.
the lady also checked my boarding pass but let me go cos its SQ but JFK gate agent are very very very "anal"