5-3-3. Additional Reportsa. The following reports should be made to ATC or FSS facilities without a specific ATC request:1. At all times.(a) When vacating any previously assigned altitude or flight level for a newly assigned altitude or flight level.
Notice it says ALL times. Not only when in non-radar, not when operating under PD, not when frequency congestion is low.
Seems pretty clear to me. Go read some article by Don Brown on avweb.