Default values can be set via configuration for all replaced objects. For example if I want to set default Account for Orders I create replaced object for Order and and specify it in default value field: http://prntscr.com/qqg9kp. it can be selected from existing value, system setting or variable: http://prntscr.com/qqga4r. Please note that if selected default value is removed from the system it can lead to different issues so do not select values that can or will be deleted.
