Setting all new customers you create to be on account is controlled by a permission.

1. Visit Maintain->System Admin->User Group
2. Select the user group you wish to Edit
   *you will need to enable this permission on each user group that creates customer files
3. Expand out the section "3.00.000 Maintain Card File"
    
4. enable the permission "3.10.135 On account customer always selected"
   *You may also wish to disable "3.10.130 Allow to change a customers On Account status" to ensure users can not modify this status.
5. click "OK" at the bottom of the page

All new customers you create will now be set as On account customers.