Vet tech in NH here: most vets will recommend year round preventatives because you never know when the weather will reach a temperature that they’ll come out of dormancy (ie: the crazy ass weather we’ve been having). This includes heartworm prevention too, as mosquitoes can be as resilient (I’ve seen them in my home a few times mid winter) and hw prevention only stays in their system for a few days to kill off any possible baby worms (microfilaria). If you skip a dose or two and they managed to get it in the interim, your dog will have to be treated for it and it’s NOT cheap like in the south….

TL; DR: year round prevention is best. For both fleas/ticks and heartworm

Bonus: most preventative companies will compensate you if you are regular about giving your preventions and they still manage to get a tick borne disease or heartworm.