Considerations:

 - How much disk space/memory can you afford? Lower fill factor will
   mean large indexes. 
 - Can you measure the current performance impact    and adjust based on
   Page Life Expectancy and Page Splits?

Keep in mind that even though the update pattern will be sporadic, a lower fill factor can come at a cost to Page Life Expectancy, because it will take more pages in memory to return the same amount of results to the application. 

Are these the clustered index keys, or just for non-clustered indexes?