I feel it's needs to be a joint decision.
SIL and BIL went for ultra sound and they sonographer write on a piece of paper what gender the baby was and have it to BIL who really wanted to know. He kept the secret really well.
Both time with mine I could tell if it was a boy or girl during the scan even before the sonographer said anything. It's quite obvious if you know what you are looking for. If you do a bit of googling and know what you are looking for you might just pick it up at your 20 week scan, that way you are not lying it hiding anything