Some say you should stay in bed for atleast 20 min before rushing off to the bathroom. I can't really say it will or wont harm your chances. I think really it's about gravity, because sperm have to swim up and if you are in an up right position, obviously gravity will pull things down.
However, sperm should know where they are going and right after they are ejaculated they start their journey up and find their way into your cervix and protect themselves in your CM (EWCM is the best environment for them). What comes out of you after BDing is usually the semen and any sperm that weren't strong enough to make it to the cervix. Sperm that are in tip-top shape should be on their way looking for an egg!