A client has a few key accounts that they do not want to change during...

Balance prioritizes equal distribution of the selected attribute across territoriesin your focus. This is the best option for ensuring revenue is distributed as evenly as possible across the entire sales team1. References: Optimization in Salesforce Maps Territory Planning

Removing those accounts from focus orlocking those units to the area are two options that a Consultant should recommend to prevent changes to key accounts during optimization in Territory Planning. Removing those accounts from focus means that they will not be included in the optimization process, while locking those units to the area means that they will not move during optimization, but the value of those units will be considered when creating balanced areas2. Leveraging a legend filter to remove those accounts or removing those accounts from the data set are not recommended options, as they will affect the accuracy and completeness ofthe data and the optimization results.