It is inevitable; one day you will create a Crystal formula or running total and it will return NULL.
However, management typically likes to see zeroes on reports, especially where user activity is concerned. A NULL value will print as nothing (i.e. blank).
You must edit the Number formatting of the field in question.
1. Right-click your field.
2. Select "Format Field".
3. Click the "Customize" button.
4. Look for the "Show Zero Values As..." drop-down on the lower right-hand corner.
5. Select "0" (zero) from the list.