I was once with a tov when someone asked him about finding out before. He said that it makes problems with the Bracha hatov vihameitiv if you know before. He said from Butchacher that no Bracha for girl since you always knew it would be "at least" a girl.
So he was bringing the Buchacher to say that you actually shouldn't make shehecheyanu on a girl, or just to explain why you don't say hatov v'hameitiv? If it's the former, we don't pasken that way (at least I've never heard of anyone being told not to make the shehecheyanu), and if it's the latter, there are many explanations given for the difference in bracha between boy/girl - even ifyou say one doesn't apply that hardly means you don't say the bracha.
Also, I don't know the halachos of hatov v'hameitiv all that well, but for example by wine, you make the bracha when you drink the better wine, even if you knew you had it all along. I can't imagine why knowing you're having a boy would be a reason not to say hatov v'hameitiv.