The best practice for filling in missing data such as franchise accounts is to implement a data enrichment package from AppExchange. These packages can automatically update and fill in missing data by pulling in information from external sources. According to Salesforce documentation, data enrichment tools on AppExchange can help in identifying and adding missing account data, thereby ensuring that the sales reps have the complete and accurate information they need. This method is more comprehensive and efficient compared to adding external IDs or using a data quality dashboard, which primarily provides analysis rather than enrichment.