When baking light maps using mental ray some of the built-in texture filtering features in Maya can be of great help. For example, when a texture bake operation has been performed, go to the File Texture node in the shader network (by default connected to the outColor channel of the assigned SurfaceShader) and under the Effects tab in the Attribute Editor adjust interactively using the Filter and Filter Offset controls (the Filter Offset control is very sensitive and is effective with values in the 0.05 range) to get the required smoothness. This may then be converted to an unfiltered texture by using the ConvertToFileTexture procedure.
To bake vertices with mental ray for Maya
For a description of the options, see Lighting/shading > Batch Bake (mental ray) > .
To bake a texture
For a description of the options, see Lighting/shading > Batch Bake (mental ray) > .
In texture baking (Occlusion color mode), turning on Final Gather may provide performance benefits.