to answer this question for me personally, I had no gender pref. with DS 1 and 2 (actually wanted a boy with DS 1 and just lucky and got him, but would have been happy with a girl too) and I didn't find out with them. I thought DS 3 was a girl. I would have bet a million dollars on it, and I am SO thankful I found out with him because I had the time to adjust. I would have hated to be crying and stuff in the delivery room and I know I would have because I was so convinced I was having a girl. Not only because of GD but also because i would have had another 20 weeks to make an a$$ out of myself telling everyone how I "just knew" it was a girl. With DS 4 it was an accidental pregnancy when I'd taken a month off swaying and I needed to know. With DD, again I just "had a feeling" it was a girl but still wanted to find out esp. after what had happened with DS 3!!
So, long story short, I personally think it's better to find out when you have any very strong gender preference for either partner. I think men often get short shrift with their gender preferences and very little consideration so I'd try to put myself in his place and imagine you may be having your 4th son and desperately wanted a daughter - it really can help to have that down time before the baby arrives.