I've also heard that an orgasm helps to suck the sperm into the uterus. I never worried about that variable, though. I figured enough sperm would find their way in, and, like I said last time, there's no way I'm going to be able to come at just the right moment. And feeling like I'm ruining my chances to ever have a child is sure going to make it completely impossible. Just thinking I might get pregnant makes it impossible for me to come. SO. What's the story? Does it matter?
As you might guess, there's not a ton of good research on this question. Would you volunteer to participate in such a study? HELLS TO THE NO, say I! But here's what I was able to find.
The first place I could find any mention of this phenomenon is in a 1972 paper that demonstrates a pressure gradient between uterus and vagina just after orgasm, which, the authors suggest, might "allow for the insuck of the cervical mucus in which the spermatazoa have been trapped after ejaculation" (p. 249). They suggest that this effect is probably only important in cases where the male has a low sperm count.
A more recent paper says the insuck hypothesis is "contentious", and suggests that while female arousal has physiological benefits (i.e., getting lubed up is good: "...the enhanced blood ﬂow to the surface capillaries increases the oxygen partial pressure of the hypoxic vaginal ﬂuid and the buffers of the transudate help in the neutralization of its normally acidic pH. Both these changes facilitate spermatozoal metabolism, motility, and survival." HOTTTT!), it's all about the vaginal tenting. I'll just include that part of the paper below, as it's quite interesting. (Click to see larger, obviously...) They suggest you actually want to keep the sperm OUT of the uterus initially!
So here's what I conclude: if you have no problem having an orgasm during or after baby-makin' sex, don't change a thing, girl! But if that's not how it works for you, don't change a thing, girl! (So yes, Twangy, it's a LIE!)