Hi Bill.
Indexes are best used for fields that are 'mostly unique' -- like a customer account number.
The great thing about the most recent versions of CMOD is that you can turn indexing on or off at any time. So, while I would normally recommend indexing date fields, you can try running queries with it on or off and compare response times. Just be forewarned that if your App Group has a lot of documents, building the index is non-trivial, and can take a considerable amount of time depending on your CPU speed and IO bandwidth.
Hope this helps!
-JD.