I would say that either is fine, depending on the style of the room and your preference. Sleek and minimal: no frame. More ornate and decorated (or with a theme): yes frame. Also, more mirror means more light and more "views", something to consider for dressing and primping, especially if you don't have another large or full-length mirror nearby.

Keep in mind that frames can be more difficult to keep clean. Heavy usage: no frame. Powder room: yes frame.

http://houzz.com/photos/4844540

http://houzz.com/photos/24509212

I agree with the above poster. We have a very traditional powder room that we put a full wall mirror on and we love it. It helps to open the room up. We chose to have our mirror resemble a large window. It really is quite a focal point of the room.