Ample's Gimp link is the way I work around the issue generally: 1 or 2 extra pixels surrounding each tile. In the created gutter the outermost line of pixels immediately neighboring the border are duplicated. That's pretty much it. :)
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Really great addon! Are you planning on adding more features? I would like to suggest a function to remove tiles in build mode, perhaps something similar to the "s" cursor snap key (Perhaps "d" and a left click).
Also, you should post this to the Blender Artist forum if you haven't already!
Anyway, keep up the good work!