"Finely chopped chocolate", melts more easily and has less risk of splitting or scorching than cooking with larger pieces.
Chocolate is, however, very brittle and chopping with a straight edged knife will result in a messy (albeit tasty) kitchen floor. Grating, on the other hand, will leave half of the chocolate melted on your hands and most of the rest in the grater.
To chop effectively, use a serrated bread knife and cut diagonally from one corner of the bar. When the edge gets to about 2 inches long, turn and do the same on the next corner (short edges work better). This is even easier if the chocolate is refrigerated for 10-15 minutes to make it more brittle. If finer pieces are required, the shards can then be chopped using a standard cooks knife.