Kenneth Fisher shares his rules of thumb with regard to indexing:
Here are my general rules of thumb, although of course, you should always use your best judgment. Also, this is for OLTP systems (ex: data entry systems) not OLAP systems (ex: data warehouses).
No Clustered Index: You really should add a clustered index. Clustered indexes are important for a number of reasons, so if you don’t have one spend some time, figure one out and create it. When possible I like dates as a clustered index but of course, it’s highly specific to the table you are working on. There are a few types of tables, some load tables, for example, were not having a clustered index is appropriate but they are few and far between.
Indexes are like cookies: you can’t have too many. No, wait, that doesn’t sound right…