It depends. What other columns are being returned by the query? What is the selectivity of the column(s) in the index?

As a rule you should not place a separate single column index on every column in a table. It creates more overhead than it is worth. You need to analyze the queries of the database and then index appropriately. A single composite index will likely be better (more selective) than 2 single column indexes and will reduce IO's. FOr example a person search on last name and first name will be faster if the index is a composite index including the last name and first name columns than 2 indexes on last name and first name.