Item Tracking
If you turn Item Tracking on for your items, it is because you want or need to be able to have traceability of your products.
Choose an appropriate time to do so because you will have to know when your traceability begins and that before that date there is no traceability at all.
You may have a legal requirement that says that after a specific date, traceability will be mandatory for the kind of items you sell or manufacture. If this is the case, that date will probably be the right time.
If this is not the case, or you have a period of time to implement it, you will have to choose a specific date. The beginning of a fiscal year or the beginning of a specific month are dates that are easy to remember for anyone. They could be good candidates.
But you also have to take into account that turning Item Tracking on, especially if it has to be done for a large number of items, or if you have a lot of data related to your items or a lot of pending documents, is something that...