It really depends on the kind of person your boss is and your relationship with him. If you're not that close to him, a letter is good for several reasons, one, that he/she will have you on file and can always refer back to it if needed for reference. 2nd is just for logistics purposes, ie. For doing your T4, issuing last paycheque, etc. A written notice is also one of your last impressions.

If you and your boss are more like "friends", or he/she is someone you think you'll probably keep in touch with after quitting, then writing a letter can sometimes be skipped. If you've verbally told him and he's cool with it, then that's ok. If I were you I would just ask if he'd accept a written letter as well, and see what he says.