In a Spreadsheet, when you delete data from a cell using either the delete key or command-X, all the formatting for that cell is lost (bold, borders, etc).
If you update data (replace data within a cell with different data), then type either return, enter, or go to another cell via the direction arrows on your keyboard, the cell retains its' formatting. Only deleting all the data in a cell results in lost formatting.
Note: This is not the case in ClarisWorks 2.x. In ClarisWorks 2.x, when you delete data from a cell using either the delete key or command-X, only the value within the cell is deleted. You have to use the Clear key (from your keyboard), or the Clear command from the Edit menu, in order to delete both the formatting as well as the cell's value.