unless you need to leave the city - like heading to the outlets in maine or new hampshire (tax free shopping) parking in boston is expensive and you'd be paying for a rental that you're just parking. as well as driving in boston is an adventure -boston, hands down, has the most aggressive and worst drivers I've ever seen - and I've been to a lot of places
Public transport is convenient and walking is the best way to go. cabs are readily available if you've walked your feet off