Do note that you might use the Contract feature for a client.
The Contract feature allows you to select rates that have been checkmarked of the type Contract, and force so that the client only ever has these rates selectable.
-In the Rates grid, create a rate, and checkmark it as a “Contract Rate”
-Now create a new Contract - perhaps even labeled with the client names in it
-From the drop down list under Contract Rates in the contract entry screen, select that contract
-Checkmark the “Limit to contract rates only” and save that contract
-Open that client’s entry screen
-Select that Contract and set the Contract Expiry Date for the future and save the client
-Now create a new workorder for that client
-Open the Labor subscreen in the workorder - drop down the selection of Rates - you will see that only the rate you selected in that Contract will display as selectable.