Reet even though DH and I have been ttc 2years now we have and still do make sure we least 1 BD so at least I have a chance of been pregnant every cycle. The least we have done it was only once about 3 days before O but we still figured there was a chance (DD was concieved 6days before I O'd so I know 3days before is possible even if not ideal) My second m/c 12months ago was concieved on O day in a motel as I drove there as DH was working and it was easier for me to visit him then he to come home. It was 300kms from home lol.
We will continue to do this even if its just once in the fertile period so we still have a chance every cycle even if that chance is very small (its small anyway with lots of BD)