I've done several metal roofs, and yes, you screw next to the rib, the part flush to whatever you are screwing into. Also, because we are prone to high winds, we always put a washer on each screw before putting it in, the bigger they are, the better it supports the metal roofing material if the wind blows. If you are totally worried about leaks, you can take a tube of Henry's roofing patch up there, and a caulking gun and shoot a dab of it before you screw, and just screw down through it and snug the washer down over it. It will not leak. Overkill probably, just depends on how tight you want it and how your weather is.
