Most all shrouded heatsinks work better drawing air away from the heatsink. Some overclockers want to keep the mainboard as cool as possible, and drawing air away from the heatsink also cools the board.
Some heatsinks work like crap in rreverse mode, while some work better. My take on it is that if you don't have heat issues with the motherboard, and you don't have a shrouded heatsink, they you should try it blowing onto the heatsink. You could slo run a test, in each configuration to see which suits you best.