I do pencil and paper, because I erase a lot. I will use a pencil and notebook if the wireframes are part of an intimate conversation with one or two other people. I also do wireframes on bigger presentations surfaces such as chalk boards, dry eraser boards or glass partitions. Sometimes it helps to leave wireframes up in public view and stand back and revisit them together from time to time. I make heavy use of the drawing tool in Google docs for wireframes when I am working on alone. I can cut and paste the 20 aspect-ratio boxes much faster than I can draw them freehand. Some types of digital are even faster and more disposable than pen and ink. I find that a sketch book lasts me much longer than it use to. I recently threw out a stack of sketchbooks going back to the 90’s. I travel a lot.