Magnetic fields are not a god friend of watches. If not careful these could make you go to your local service center more often than you wish and have your precious watch demagnetized. This is not the worst thing that could happen yet it could drastically alter the precision of your watch. Definitely a pain to go and have that done all the time. Because most people are so "mesmerized" by looking at the back of their movement, manufacturers make sapphire backs more than ever. (Really don't understand why. Most are equipped with the same'ol movements, and they look like shit anyway!) Sapphire backs add to the momentary delight of most gullible newbies out here... but does not help when protecting your watch against magnetism. Nothing wrong with ugly movements, the Lemania 5100 is amongst the most unpleasant ever designed, yet it's one of my all time favorite movements. ETAs, Valjoux, Unitas, etc... are indeed good, proven and reliable movements, yes. Nevertheless, they still look like crap. Needless to say that most of these are so poorly decorated that they are a torture to look at. If I had the choice between looking at these or extra protection against magnetism I would choose the latter. Hell, given the choice I would simply ask not to see them at all! Incidentally, here is a browny point to Rolex, they never budged. God knows I give them little love but credit is due where credit is due.
People with jobs near highly magnetic sources such as Doctors in Hospitals or Lawyers and Pilots that continuously cross metal detectors should consider watches such as AP's OffShore, the Ingenieur by IWC, The Pilot series by SINN, the Milgauss by Rolex etc... Except for a few exceptions all of these have movements encased inside a special antimagnetic case. This is located between the movement and the actual watch case making it impossible for you to see the back of the movement, obviously. If you have a boring life and enjoy good health, by that I mean; no travel, no court visits and no trips to hospitals close to magnetic scanners, then you should be alright, unless you have a music system that consist of a set of monstruous Wilson Watt/Puppy powered by two gigantic monophonic KRELL amplifiers (each with the power requirement of a third world county). Yes, the kind that causes your belt buckle to lounge towards your speakers every time you turn up the volume. Yep, at the same time your local Power Plant Manager is perplexed at the sudden drain of a "gazilion" watts, for no explainable reason. Then it is likely that you are dead anyway since the speakers probably removed all the iron in your body by now.
But wait! There is one source that is often overlooked. Do you cook? Are you one of the new wave of self made master Chefs? By any chance, do you happen to own one of these super modern INDUCTION kitchens? Yes? Then you are in trouble. The main principle behind induction is magnetism. If you have a compass at home you could try and bring it close to your induction plate and see if North is still where it's supposed to be. Needless to say that I cook without my watch on my wrist, recommend you do the same.