Good question. To be honest, I can't understand the increase in magnetic field strength with altitude - how? The earth's field gets weaker as you move further away from it. I doubt very much that there are enough field generators in an aircraft to affect a watch that much.
Example: during my physics days I frequently did experiments with a 2 tesla magnet (info: it required water cooling and 10,000 volts to run) - for those who have no idea how strong that is, it's about 4,800,000 a/m or something in that range. But the field strength anywhere immediately outside the 1cm square area between the poles dropped dramatically, to the point that when you were 20-30cm away, it was only maybe 2,000 a/m. A fair quantity of energy is required to generate field strengths like that over large areas of space - which makes me wonder how necessary it all is.
And with regard to modern aircraft, you would have expected the shielding to improve, right? Magnetic fields aren't all that good for your body either, and you certainly would have thought that the aircraft manufacturers would have done something to address this if it indeed represented such a risk.
But in any case, I'm not worried. Ever since I got the Dreadnought I've always flown with it, and the Omega Dynamic (which also has antimagnetic protection, to 9600 a/m I think) before that. :D
Just my two heavily devaluing malaysian ringgit...
Ming