I happily sit 7 feet away from a 90" screen when viewing 3D on my 720p projector. Any closer and the frames of my 3D glasses would be cutting off the edges of the image. At the same distance in 2D I can see a screen door effect and the 720p image is clearly lacking resolution, but in 3D, it magically disappears.
The only minor issue with sitting so close is I can sometimes see some ghosting when looking through the edges of my glasses. If I were to move back more of the image would be seen in the center of the glasses where there's no ghosting, but I'd rather just sit close and move my head slightly when I really have to!