I found out at the 20 week scan with all three of my boys, and was disappointed to varying degrees each time. With DS1 I was sad and cried about it for a day or two, but then quickly came around because it was my first and having a baby was new and exciting. DS2 I'd found this site and swayed pink for, and between him and DS3 I'd done two MS/IUI cycles and not gotten pg then did a very thorough sway that resulted in a discouraging chem pg before finally getting pg with DS3, not to mention I knew I was limited because I ended up being a c-section and thus can't have a dozen kids, so you can imagine I was increasingly upset with the gender reveal each of those times.
But do I regret finding out? NO!!!!! People say "don't ruin the rest of your pregnancy," but I would SOOOOOOO much rather work through those awful feelings pre-baby. For me figuring out an awesome name has always helped, and if I still had been hoping it might not be a boy I don't think I'd fall so deeply in love with the name and get so excited. If I'd waited, yeah I'd probably be happy and full of nothing but pure love at the birth, but once those post-birth hormones wear off, I know GD would rear its ugly head (especially with my ppd-ish tendencies), and it would be so much worse because then the guilt would be so much worse with that innocent baby in front of you. I've seen it happen on here many times before - women waiting for the birth, then finding out it's not what they hoped for and saying "I don't care I just love him so much," then two or three months later they're back on here talking about GD and planning their next baby.
You know yourself the best, so if you think you'd only be mildly disappointed and quickly get over it 100% if you wait, then by all means wait. But if you think you're going to have a tough time of it for a little while, just remember that those feelings usually have to come out sometime somewhere.