I've tried using a magnet, and found that it will pull the strings into its plane. I do use a weak magnet to level upright strings, but place it within 10 mm of the V bar to minimize that effect.
A bubble gauge does need to be stabilized, as in this photo, where thumb and finger are just barely touching the level while the strings are plucked. That's why I don't like the Mother Goose tool (copied from Fazioli's): it is short, and you often need to reach down behind a strut to stabilize it, and lean over to try to see the bubble. A level must be placed as close as possible to the strike point, which is usually under the very front edge of the dampers.
Regards,
Fred Sturm
"Travel is fatal to prejudice, bigotry, and narrow-mindedness." Twain