Safety pilot is non-logable time if you are the third pilot on the flight deck of an aircraft certified for 2-pilot operations.
There is no real need to get your logbook countersigned as PIC/us after each flight (it does get a bit of a bore after a while). The only reason is to show that you have the time when you need to unfreeze the ATPL. Generaly most people obatin a letter from the pilot manager / chief pilot stating that all the time is as required by the CAA