He'd rather keep Cersei's daughter in Winterfell, in case Tommen won the war in the South. If she was trueborn, Myrcella could even be used to lay claim on Casterly Rock. And if the rumours were true and she wasn't... it didn't matter now, did it? The only person that could say for certain was Cersei Lannister and he doubted the former Queen would shout it out for the world to hear.