The edges where UV seams are could be split/hard /sharp since a game engine would split such edges anyway and edge vertex count will be doubled at such edges.
Any other edges should be smooth/soft and making them hard where no UV seems are just makes extra redundant vertexes to calculate and troubles for normal map baking.
If adjacent faces within same UV island are 90 degree to each other it should rather have bevel /chamfer with its vertex normals edited/rotated perpendicularly to those faces . It's necessary to get rid of smooth shading gradients on what supposed to be flat surface.
There is no problem to make hard/sharp edge looking beveled or rounded by means of normal map only (if necessary) . It just needs a special approach. A support edge loop along that sharp edge with perfectly perpendicular vertex normals. After normal map baking such loops must be deleted.
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