Wouldn't a magnet cause the shirt to pull towards the combadge just before the badge is affixed? The combadge is being held by a much strong force than the costume, the human hand. Sure a weak magnet could be used, but would it be strong enough to hold the combadge in place after that?
When I worked in a restaurant, after about a week I was able to put my name tag on without even thinking about it, without looking and it was always on straight and in the approximate spot as every other time I put it on. Rarely was it was askew or noticeably higher or lower.
On set I'm sure the actors had little difficulty putting the combadges on properly and held with a something, a light glue perhaps, to the uniform. Good editing would allow for the propmaster (is that the word I'm looking for?) to set the badge precisely and attach it in a more secure fashion. Removing them would also be the same, except the editing allows for a less secured combadge to be affixed to the uniform for easy removal by the actor. Editing, it's a good thing.