[dba-SQLServer] Does building cover indexes pay?

I have processes which process huge tables (65 million records are common) and update specific 
fields with the results of user defined functions.  For example I have a function which builds a 
text "code" using four passed in values.  These four values come from four fields of the record and 
the resulting code is placed back in another field of the same record.

ATM I build a "cover index" just before updating this field, perform the update, then delete the 
index when done.

I am wondering whether you guys believe that the time to build the cover index will be recovered by 
less time taken to perform this update.  IOW does it take sufficiently less time to get data from an 
index vs a table scan that it is worth building the index?

-- 
John W. Colby
www.ColbyConsulting.com