Depending on what your intent is when creating the tileset, you may want to have a snow/grass combination texture. Additionally Glest supports multiple textures for a single surface, and you can even set the frequency that a particular texture will be used. If you look at the forest tileset's XML file, you'll see that the texture with a stone in it is used very infrequently, only a small percentage of the "grass" texture will use that image.
If your goal is a melting winter, you'll need a grass-to-snow texture. I don't see anything wrong with that.