Most guys, myself included (well, back when I kept a log book anyway), log all IRO, IO, IRP (whatever your company calls it) time as SIC.
If it was a 10 hour block day and I was the IRO, IO, IRP, for that day, I logged all 10 as SIC.
You can argue that I shouldn't have been logging SIC time while I was in the back sleeping and eating meals. You can also argue that the captain and FO should deduct the time while they were in the back sleeping and watching movies. Does anyone bother to do this, or even care? No.
As the one poster above said, the big key here is do NOT log any time as PIC if you are not a captain with your company. That will get you thrown right out of any interview you ever plan on attending.
I flew for one particular company that used FOs and Captains to fly as IO's. I never did ask one the Captains who was flying as the IO if he logged the whole flight as PIC, or SIC, or half and half, or what.
You just have to ask yourself one question. WHY am I keeping a logbook?
If the answer is because you are trying to get an interview somewhere else, then play it safe and log all IO time as SIC, even if you happen to be a Captain at the company.
If you actually want to know, for some ungodly reason, how many trips you flew as the IO, then create a separate column.
If you want to show some other dorks that you actually "Sat in the left seat of a 767-400 for 3 hours while the captain was in back sleeping" on such and such dates, then log it as PIC. Just don't plan on going to an interview with that.