Reb Yaakov Emden in his sidur has a section on marriage and goes into great length to say that sex than can bring children should be only Friday night after the middle of the night because that is when holy souls come into the world.
But at any rate, I would like to suggest this practice to people that wish for good children. This might not be a guarantee, but I believe that this certainly helps. And knowing how hard it can be to have good children, it makes sense to do what ever can help.
