previous move before the current

everybody talks about looking moves ahead but during game how many of the opponents previous moves do you study during game.
To be able to tell if the opponent has a plan with what he is doing. I myself look at the 4-5 previous move sometimes I think I am able to tell the opponents plan.
Does good players look at/analyze the previous move or is it wrong focus.  

IMO it's a bad habit and at the very least a waste of time and energy to look at previous positions.  And what if your opponent's plan is wrong?  Will you still work to prevent it?  As estragon said, all that matters is the current position.  Decide for yourself what the important aspects of it are.