Demi, sperm will move all by themselves with no help from you whatsoever. Women have been getting pregnant for thousands of years (or more) and have found that sperm seem to be able to find the egg on their own. If you are healthy, and the man is healthy, putting your legs up is not necessary. BUT if it makes you feel better, it will not hurt anything either! GOOD LUCK!
No this is a myth and hasn't actually been proven to make a difference. Many people think it does - including doctors but there is no evidence it helps. Sperm gets to where it needs to when the man ejaculates without the need to be doing things like that.