Entirely depends on the goal. From what I understand ref. @cdeil the `sum` method is aimed at being quantitative. In this case you definitely want to use zero padding, or you change the total sum of the image! This is really bad for scientific applications. If this ended up in `skimage.measure`, or a wrapper for that method, I would definitely say zeros is the way to go.

On the other hand if you're going for looks rather than quantitative science, "downsampling" an image with local means, then I would say reflective padding would be appropriate - otherwise you'd get dark edges.