When I had mine I saw her parts to reassure me it was a girl. Mind you its very rare to get thevsex wrong once let alone throughout all scans in the pregnancy
In the uk at the 20 week scan they give it a 90% chance that its the sex they say so there is a 10% chance if it being incorrect. Im 25+5 and ill be having my 4d scan at 28 weeks and I will be asking to double check the sex as I also have brought loads of girls clothes. Its your choice if you have a 4d scan.
That's what I was wondering as well because first my tech wasn't sure. Because my daughter kept her legs closed....but then she opened them not really wide but wide enough where it looked like a split and not a weenie sticking out lol...but the tech kept saying she was sure it was a girl...so I'm crossing my fingers not to get a surprise because I have already started buying girl things.....
Keep tags on everything, if its not a boy you can return it. Like Rachyou said it's rare for it to be wrong, and most techs wont tell you unless they're very sure, if you're worried you can always buy some pink or neutral things as well!