We have found out all 3 times. Lots of reasons.
-I am very impatient and DH is an engineer and really likes to be organised.
-We are both scrooges when it comes to money so I wouldn't have been able to spend money on pink or blue stuff without knowing the gender, and didn't want a whole wardrobe of unisex stuff.
-In my opinion the birth is surprising enough, this way I get a surprise half way through the pg, and then I get the surprise of meeting my baby at the end. And TBH, checking the gender of my babies when they were born was the last thing on my mind, I just wanted cuddles.
-This time though there was the extra consideration of DS1. He is old enough to understand sister vs brother and was adamant that he wanted another brother. I had considered not finding out this time, but changed my mind as I didn't want DS1 to have to deal with gender disappointment when the baby arrives. Lucky I did find out cause this one is a girl and he wasn't impressed when he was told. It hasn't taken him long to come around, but I am glad I wasn't dealing with his shock at the same time as bringing home the new baby.