Not sure what you mean by "Letter of Employment".
Per the current CEC Document Checklist, letters of reference are required, and without them (and the required information), your application will fail.
Prior to the Jan 2012 revision of the checklist, the required letter was called a "Letter of Employment" (not letter of reference.) Name change for the same document - just as critical then as now.
So it depends what you mean. If you have the required letter of reference and it contains all the required information, you do not need any additional letter.