If I understand Newton's law (I'm always forgetting which one) it is that every action has an equal and opposite reaction.

I figure that means it makes no difference for compression where you cleat the main halyard. The loads on the halyard are no different, the mast must therefore provide the equal and opposite reaction. The only things I can see this affecting is mast top cleats will reduce halyard stretch and may help induce mast bend particularly in combination with rotation (but I'm not really sure on that).

Dennis