Always worth checking in online for a few reasons:

1) BA know you will definitely be travelling which increases any possibility of an upgrade.

2) If in the unlikely event of downgrades being required you would be safer than somebody who's not checked in.

3) If for any reason you get held up on your way to the airport, we can print a BP after the flight has closed if you've checked in. OK I realise this won't work at LHR where conformance is in play, but this has helped passengers at LGW quite often in the past.