If that possibility seriously concerns you, follow it up by physical destruction of the drive. Magnets strong enough to actually damage the data aren't a good cost/benefit from what I gather (too strong, too dangerous, too much hassle), so either shred or melt the drive platters.
I'm generally satisfied with df if=/dev/zero of=/dev/sda because I'm not actually dealing with anything that sensitive, but if you are, just take appropriate steps. Defense in depth.
It doesn't concern me that much, but I'd rather fill the drive with random data myself first. I can do the ATA-erase thing afterwards.
On a related note, I also have zero trust in manufacturer-supplied encryption that sits in the drives. There is no way for me to verify whether it actually does anything.
I'm generally satisfied with df if=/dev/zero of=/dev/sda because I'm not actually dealing with anything that sensitive, but if you are, just take appropriate steps. Defense in depth.