In my case it was absolutely necessary, since I finished undergrad five years before I applied. My old professors and PIs were thrilled that I was applying and happy to update letters they had written. Some of my interviewers commented on the "lasting impression" I left on faculty, which worked in my favor. But I did hear that letters should be as recent as possible; I ended up using my alma mater's premed committee (at a feeder school) and they strongly advised students ask for updated letters.