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The best approach to follow will be easy to see if you understand how the UI labels system works.

Bottom-up point of view
The basic concept of the multiple UI label property files is this: start off with the most generic, most often used labels, then build upon that with application-specific labels. The CommonUiLabels.xml file contains labels common to all components. The EcommerceUiLabels.xml file contains labels specific to the eCommerce component. Doing things this way reduces redundancy and it allows an application to redefine labels found in the common (or shared) label files.

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The bottom line is, it is better to add translations to property files than it is to delete labels. Deleting duplicate labels makes sense at first glance, but problems could come up if there was a specific reason the duplication was needed (for instance to avoid an inter-application dependency. For instance EcommerceYourShoppingCartEmpty may be present in OrderUiLabels.xml to eliminate the need for the Order Manager component to load/use the ECommerceUiLabels.xml file.)

Here is also an Top-down point of view
An interesting explanation from Adrian : https://issues.apache.org/jira/browse/OFBIZ-1698

Exernal point of view
This link may be of great interest too : Standardizing and Internationalizing OFBiz User Interfaces . Beware : the part about ".properties" files is deprecated in OFBiz since February 2008 (we use now XML UTF8 files), still valid in release4.0 though)