I prefer to turn the lights and ignition switch on with battery disconnected, but it's all the same really. I did some tests years ago at an electronics company I worked for, we found we could take some static RAM chips down to less than a volt for several minutes and they would come back up with their data still in them. All it takes is a couple of decent capacitors and no load to discharge them and you can keep the memory chips powered for a remarkably long time. Always worth a try if all else fails. Shaun