Hey Pool,
as far as I know the captain is authorized by the company to apply more conservative margins if he considers it necessary. I occasionally have used a higher thrust setting or flap setting than the optimum calculated by the OPT because of a rough runway surface, possible windshear or whatever. I think what is discouraged is unnecessary conservatism and this is not an easy message to get across. Hard to draw the line sometimes. Mind you, some of these TRIs....