I don't think any of them should be left out but I think Sissoko was bought primarily, by RB, for away matches so he could break up play and distribute the ball to Gerrard/Alonso as they are then able to start an attack. So far this season that is exactly what he has done and furthermore he done it the way you would expect a seasoned professional to do it.
Even though I don't think any of them should be left out of the team, I do believe it is in the best interests of the team if a match can be won without one of them then they should be left out. My reason is we have already played 11 matches (only three of them in the EPL) so far and there is going to be a lot more before the end of the season. We should all trust Rafa as he has so far steered us in the right direction, which has brought success to LFC and long may it continue.