To avoid tear-out, or splinters in the finished cut, there are a few tips for router work.
Don't work your way around the edge of a board. Always cut the end grain first and use a piece of scrap held tight to the good board to hold in the splinter that wants to fly off the end of the cut. If you first cut the side grain, then go from end grain to a finished side grain, you cannot back up the cut because of the mold pattern.
When making a cut on the side of a board that tends to splinter, always make a series of light passes in the "wrong" direction. Hold the wood and the router firmly as they will want to fly. This presents the cutting blade in such a way that it pushes into the wood, not pulling out of the wood. Then make a final clean-up cut in the "right" (left to right) direction, but you are not removing enough wood now to create a splinter.