Ok Ajay.
Since 'reference' field is missing from most of the line items in GR/IR, I am OK for setting the 'assignment field' only for automatic clearing.
[I think if I set 'reference' and 'assignment' together, system will not select those items where one field is blank even though the other criteria may be OK. (i.e. both field values should be mandatory for selection and clearing. right ?)]