I don't think you need to do it often, if at all, really.
When I first started playing, I would get a little bit stressed about writing to everyone too...because I heard it helps you build friendship. Also at the beginning, they send you letters back, with items attached, and it's good for building up your catalog. But it's usually junk.
I would, however, talk to them all at least once every day...if not multiple times. They build friendship with you faster if you talk to them, and also they let you know important stuff, like who's thinking about leaving...unless you want to be surprised on a day by logging in and seeing your favorite is in boxes. :'c
I definitely talk to everyone daily! I just was dreading letters. Nice to know it's not so important.