Wow... do they really tend to use magnetized screwdrivers for that though?? If there was one part of a computer I would have thought would be most at risk or susceptible to a magnetic screwdriver it would be platters on a modern hard drive.
Modern HDD platters pack the bits so tightly together that the magnetic coating must have very high coercivity (resistance to changes in magnetization); otherwise adjacent bits of opposite polarity would erase each other! It takes a pretty intense field to change the magnetization on a modern HDD platter; the write heads are designed to create a field comparable to that of the strongest available rare-earth permanent magnets (but obviously concentrated on a microscopically tiny spot). The magnetic field at the tip of a magnetic screwdriver is going to be orders of magnitude less.
Unless you're touching the tip of the screwdriver directly to the platter (and probably not even then...), I'd say the risk of flipping bits is basically zilch. And if you're touching the platter with the screwdriver you've got bigger problems anyway.