The advice to put a pillow under your bottom after having sex and stay like that (reading a magazine to quell boredom, perhaps!) is common. Many people also advise couples trying to get pregnant to to position themselves so that the sperm will end up as close to the cervix as possible. That does seem to make a lot of sense, and many couple will intuitively feel that, the more they "help" the egg and the sperm to meet, by making those swimmers' journey to their destination as short as possible. However, there is no evidence that sexual position affects pregnancy rates.
Having regular intercourse at the right time of the month, and making sure that ejaculation occurs inside the vagina are good enough. For those who want to go that extra mile and would like to see whether they get pregnant sooner (not that there's any way of knowing, really), having sex in the missionary position or other positions that position the penis close to the cervix (try "semi yoga poses" like the women having her bottom in the air, along with her legs) certainly does no harm.