There is none, I have never done different solder masks for top and bottom for an SMD component, ever. Nor can I see any reason why re-assignments should not swap, it causes more problems than it would ever solve, it is a feature I have never needed in my PCB design life and imaging I never will.
The only possible reason may have been for wave soldering SMD components, as some liked to have hugh pads for the wave side (why, I never know, the geomatry of the solder joint is the same for top bottom, reflowed or wave soldered), but again soldering SMD by wave these days is a no no with a lot of components, and the ones you can you use the same pad size top or bottom.