I watched an interesting documentary on the female organs, sorry, but I can't remember what is was called, and they did a test where they injected a special dye into that general area, and she proceeded to give herself an orgasm and they could see on the screen the dye being drawn up into the uterus area during and after the orgasm.
So I am guessing that having an orgasm same time/just after would be benificial to getting more of the sperm where they need to be.