If you are texturing for mobile games (128 texture), do you need to leave space along the texture borders in order to avoid texture bleed? Or can you maximize your space and use every pixel?
This doesn't make any sense. Diagonal or not, if you're using mips then the model will be textured with a blurrier version of itself when you see it at a distance, which is what causes the bleeding problems. If your game is using 'point sampling' also called 'nearest neighbor' then you probably still want at least a pixel…
I think the OP means padding along the 0-1 UVs boundaries. If your engine allows you to disable texture tiling then you don't need to worry about it. If not, you need to pad like any other shell.