Should I create a multi-column UNIQUE index?
You would create the index if any of this is true: 


There are queries or DML statements that benefit from this index.
You want to enforce uniqueness of the combination of the index columns. 


You would not create the index if all of this is true:


There are no queries or DML statements that benefit from it.
You don't want to enforce uniqueness of these columns.
DML statement performance against this table is critical and the benefit of one fewer index offsets the loss (if any) of query performance.


This applies equally to non-multi-column indexes.