FAR 61.51:
(2)An Airline Transport Pilot may log as PIC time all of the flight time while acting as PIC of an operation requiring an ATP....
(f) Logging SIC time. A person may log SIC flight time only for that flight time during which that person:
(1) Is qualified in accordance with the SIC requirements of 61.55 of this part, and occupies a crewmember station in an aircraft that requires more than one pilot...
(2) Holds the appropriate category, class, and IR for the aircraft being flown...
This pretty clearly shows that a Captain can log the total time airborne if he is the only PIC on board, while an F.O. may only log that time which he is in the seat. As a matter of principle I only log the time I am in the seat even though I am the only PIC on board.
The question gets a little dodgy where IROs are concerned because they are type rated in the airplane, but they aren't the PIC. Your airline should have guidance on it. If you are really concerned just start a separate column for IRO time in the left seat in your logbook.
and just so we are clear, PIC means the person who:
(1) Has the final authority and responsibility for the operation and safety of the flight;
(2) Has been designated PIC before or during the flight; and
(3) Holds the appropriate category, class, and type ratings, if appropriate, for the conduct of the flight.
Typhoonpilot