He didn't give the nod to Ney at Waterloo, Red Michael launching the initial charge on his own. Napoleon first saw it in full career and, apparently, remarked that it was an hour too early. Napoleon did give the order for it to be supported.
I believe that if Grouchy had been there as the commander of the Cavalry Reserve, the initial charge would not have happened.
De Brack has some interesting things to say about the charge of the Guard Cavalry at Waterloo. The comments are in Ron Pawly's excellent The Red Lancers.