Do magnets dull knives?

The magnets themselves don't dull them; they're not nearly strong enough to deform a blade.

But it is possible to damage your knives on those racks. It's very easy to drag the blade along the rack a bit as you pull it off, and easy for the magnet to snap it against the rack as you put it on.

Both are avoidable, though. When removing the knife, you twist it so that the sharp side leaves the magnet first. When putting it on the rack, you let the blunt side touch first, so when it snaps the rest of the way on, it's only the flat side against the rack, never the blade.

It also helps to get a version that's not just bare metal; there are plenty with the magnet underneath wood. It might not be ideal if you let the blade drag along it a lot, just as you probably don't want to drag it along your cutting board, but it's a lot better than metal. Also, since the knife isn't directly against the magnet, the force doesn't increase as sharply as you get to the rack, so there's less snap.

So, if you're worried that you (or guests in your kitchen) may not always be too careful, a bare metal magnetic knife rack is definitely not a good idea. But unless you're really protective of your knives, the wooden kind is probably fine.

Folks have also pointed out that in principle, long-term use of a magnetic rack could magnetize the blade, causing it to pick up tiny bits of metal, especially during sharpening, which would then wear and dull the blade. As far as I know, this is not a big deal: I've seen knives stored long-term on magnetic blocks without noticeable magnetization. You could always keep an eye out and demagnetize the blade if necessary.

There is some research going on on how to measure mechanical properties of steel using its magnetic properties. That might mean that if you change the magnetic properties of the steel (like hanging it on magnets), you could change the hardness of it - but I haven't found any article on it to support that.

A problem which might occur later on is during sharpening (on a stone, not while using a sharpening steel):

The knife will probably get magnetic over time. When you try to sharpen it, the small particles will stick to the blade, making it harder to sharpen it. So it might seem to be dull because you can't get it as sharp as before.

To get around that problem you can try to knock it several times against a wooden board (back of the blade). This will demagnetize the blade and sharpening should be easy again.