The crappy workaround I found myself having to do at times, is to create a copy of the material's image texture. There's no way to "snap" to incremental degrees when rotating the material map, which is mega frustrating for textures that necessary to line up with object edges. That little 3d icon with the 3 circles you use to rotate the map? Yea I hate that thing. No end of trouble. So instead, I'd often manipulate the image itself, in GIMP (or Photoshop if you have it) or even MS Paint, and just roll with that, for more precise control over the map.
