Script that runs on a sublist only the first time a line is added?

I'm creating a simple script that upon entering a "item" line item in a sales order, the scrip checks the main sales order location field and applies that location to the line item location field.  The idea is, if a sales person chooses Puerto Rico at the sales order level, all the items will default to Puerto Rico at the line item level.  However, I need the sales person to be able to manually change the location to another warehouse after the line item is initially entered. 

Currently, I'm using the Validate Line Function, which I thought would only fire when the line item is initially created.  However, it seems to run every time a line item is edited.  Is there another event type that will do what I need?  If not, any other suggestions would be appreciated.  Thank you.