My 2 cents worth....
For me it doesn't really matter what number is on their back as long as all the core areas are covered and the players styles and strengths compliment each other. There are obvious situations, you obviously want you're best ball stealer/quickest to be playing on the openside of a scrum. You need a no8 that knows how to control the nut at the back of a scrum.
However all the other stuff in general play and at LO time, IMO just needs to compliment each other.
Take for example one of the more successful loose forward trios of the ABs in Collins, McCaw, So'oialo. Collins played blindside after being forced to play no8 to 'fit him in', but he was not the greatest at the back of the scrum. He did his best work on the blind yet he was the main ball carrier in that loose trio - the enforcer so to speak. Rodney was the ultimate garbage man who clean out, hit tackle after tackle, ran the field all night in support and most importantly cleared the tackle for McCaw to make a tilt at the ball.
One of my favourite players during that time.
They worked very well together linking and combining in attack and defence.
All three were used at LO time, with Rodney probably used more often than the others given out propensity back them to throw to the back (we soon changed that over time and Kaino and McCaw were the LO targets). Collins played the midfield battering role.
What's also interesting is that whilst that was the number one combination of that era, Graham Henry tried a few others. And one that I saw work brilliantly at test level was McCaw at 7, Masoe at 8 and a renewed Ruben Thorne at 6. They all seemed to be on the same page but covered all bases well. Masoe was a monster with ball in hand and Thorne was used a lot at LO time freeing up McCaw to also run the ball from set piece. Thorne did a lot of the 'Rodney' role of clearing bodies and clean out work.
The other thing that is key is time. That successful trio of McCaw, Collins, So'oialo got plenty of test matches together. Sure it helped that two were provincial and Super rugby teammates, but Henry persisted with them over a good stretch of tests despite the rotation of many other players.
For me the OZ back row has been chopped and changed a hell of a lot for to injury, probably lack of form, poor timing etc, so we don't really know what combination is the best one. Pocock when fit has been the one rock, but now he is broken and there is an urgent need to find 3 to gel.