Marking & mortising gauges have conical-shaped points where the gauge marks the workpiece. The shape of these points produces some amount of tear out when marking across the grain.It also
creates a wide mark. This can be good if you have trouble seeing up close, but if you need dead-on accuracy and prefer clean marks, file a flat on the side of the point 
that faces the fence of the gauge. If you are using a mortising gauge, file the two inside faces of the points. The difference in track marks between conical points and knife-edge points is shown in the top photo.
If you have an antique marking gauge, you may need to flatten the fence on a sheet of sandpaper. The fence on my antique gauge has two strips of brass that, when I bought it, stood proud of the wood, each to a different degree. One strip was about 1/32" proud, a
nd the other, a bit more. I resurfaced it on a sheet of sandpaper that I fixed to my jointer bed until the brass strips were flush with the face of the wood fence. A dead-flat face (why does that sound scary?) will improve the performance of the gauge.It could be that the gauge was made this way intentionally, with the brass not flush with the wood fence. But by relying on two thin pieces of brass to maintain a consistent distance between the fence and the edge of your workpiece, you risk making a wobbly line when you reach the end of your workpiece and one strip of brass is no longer in contact with its edge.
In short: a knife-edge and a flat face will improve your marksmanship.
Note added 3/12: in case you don't read comments, some readers are referring to Tage Frid's book that recommends filing a slight angle on the point so it will pull the gauge against the edge of the board.