Sperm typically live alot longer (up to 5 days) than an egg (up to 24 hrs) so bding before you O is much easier timing than trying to pinpoint exactly when you O. But, it's much easier for the sperm to get where it needs to if you bd when you have EWCM, which is typically a few days before you O. Not saying it won't happen if you bd when you don't have EWCM or if you bd after you O.
I normally O on CD 13 - 14, or at least that's my guess.

For my 1st pg we bd'd on CD14 (finally - it'd been like 3 weeks

) and for my 2nd pg we bd'd on CD 11 I think.
HTH!