Ours is a game of managing errors, of keeping the ball in play, and with each and every shot always playing the percentages.
Play the ball close to the line, you encounter the risk of an error. Play the ball close to the line when the opponent is out of position, now the reward may outweigh the risk.
But worse, sometimes it is possible to lose points simply because you are never truly taking any risks.
With this in mind, consider the line of the incoming ball, and whether you return the ball back along that same line – and this play will always minimize errors. You are playing essentially as a wall. But if you change the line, such that you take a cross court and return if up the line, or you take an up the line and return it cross court – in those instances you are changing the line of play.
If the opponent hits the ball much harder (setting up a power line) then your decisions become much more important, said another way certain decisions expose you to much more risk.
I recommend always play back along incoming cross court power lines – minimize risk, let them change the line.