If you are big enough to have to worry about this you are probably a company with plenty of resources to think and comply with this. So it's hard to imagine how many readers of HN are getting their answers on HN (or similar). If you are small time nobody is going to come after you. Sure something could happen and you could also get a traffic ticket going 57 in a 55 zone and a host of other outlier events.
> AKA: be paranoid about keeping them, encrypt them, use SSL on your site, respond to requests from people if they ask if you have them, fix them if they're wrong, don't use them if they say you can't.
One size fits all advice doesn't make sense in this and in other similar cases. You will spend a great deal of time and effort dealing with 'maybe's' instead of the day to day.