That depends on how particular you are! Professional chefs proclaim that fresh basil should always be torn and never chopped. Chopping basil breaks open the cell membranes which leaches out the flavor. If your basil will be cooked in a sauce or baked, tearing or cutting won't make that much of a difference. If you plan to use the basil in its raw form in a salad, try tearing the leaves and see if you notice the difference.