We all know by experience that missing UCI # is not a big issue but here are CIC words confirming the same
http://www.cic.gc.ca/english/helpcentre/answer.asp?q=775&t=4
FAQ: I forgot my client ID/UCI. How can I get it?
This eight digit number appears on all documents you get from Citizenship and Immigration Canada (CIC). It looks like this: 0000-0000.
Look for this number on:
letters you received from CIC,
your immigration document such as a PR card, study or work permit.
If you do not have any of these documents but want to fill out an application form:
You can leave the field for UCI (Client ID) blank.
Note: If you have not dealt with CIC since 1973, you will not have a UCI or a Client ID.
http://www.cic.gc.ca/english/helpcentre/answer.asp?q=775&t=4
FAQ: I forgot my client ID/UCI. How can I get it?
This eight digit number appears on all documents you get from Citizenship and Immigration Canada (CIC). It looks like this: 0000-0000.
Look for this number on:
letters you received from CIC,
your immigration document such as a PR card, study or work permit.
If you do not have any of these documents but want to fill out an application form:
You can leave the field for UCI (Client ID) blank.
Note: If you have not dealt with CIC since 1973, you will not have a UCI or a Client ID.