In my personal experience, JetBlue is significantly more lenient than some other airlines (looking at you AA) in this area. You'll probably be fine if your bag is not overstuffed and can easily fit in the overhead. Generally if you can make it past security with the bag, you're at least not going to have to pay to check it. If there isn't room in the overhead, you may have to gate check it, but at least that's a free service.