In the chapter The Seven Potters in Deathly Hallows, Remus Lupin is appalled when Harry tells him Harry used Expelliarmus on Stan Shunpike because he felt Stan was under the influence of the Imperius Curse during the Battle of the Seven Potters. Lupin felt Harry should have used Avada Kedavra or Stupefy (if Harry wasn't prepared to take a life):
Lupin looked aghast.
‘Harry, the time for Disarming is past! These people are trying to capture and kill you! At least Stun if you aren’t prepared to kill!’
Deathly Hallows - page 64 - Bloomsbury - chapter four, The Seven Potters
¯¯¯¯¯¯¯¯¯¯¯¯¯¯‘So you think I should have killed Stan Shunpike?’ said Harry angrily.
‘Of course not,’ said Lupin, ‘but the Death Eaters – frankly, most people! – would have expected you to attack back! Expelliarmus is a useful spell, Harry, but the Death Eaters seem to think it is your signature move, and I urge you not to let it become so!’
Deathly Hallows - page 64 - Bloomsbury - chapter four, The Seven Potters
If the Death Eaters believed Expelliarmus had become Harry's signature move, why didn't Lupin, or someone in the Order, warn Harry not to use that spell, should they encounter any Death Eaters?
Lupin was very vehement about it -- after the fact. Why not warn Harry beforehand?