Override vs. Do not override for field values

So quick question has anyone ever experimented with an override vs. don’t override script for custom field API’s?

Use case:

First Attribution: Triggered from multiple sources (Google Analytics UTM, Web form, Manual input, etc) Always keep this the first value that enters

Last Attribution: Triggered from multiple sources (Google Analytics UTM, Web form, Manual input, etc) Make this so it is always the last value

Any help would be appreciated thank you!