When compiling maps with cGPSMapper, there is an option POIIndex=Y/N.
But the Garmin IMG format contains 'Points of Interest' and 'Indexed Points of Interest' as separate data items.
It would be nice to switch indexing off or on for individual items (e.g index Hospitals but not trees) - is that possible?
Not all POIs but only specific types of POIs are indexed. These types are also sorted into multiple categories and subcategores. When you choose a type for trees and hospitals, you should select appropriate ones. See some info here:
http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/POI_Types
I think poi behavior depends not only on device but on compiler too.