I doubt a magnet that small would cause any problems. I'm not sure about the new MacBooks Pros, but the old ones had an embedded magnet that I think was used to detect the screen closing.
As far as I'm aware, all Mac laptops now use magnets to keep the screen closed. Quite strong ones, really, they easily keep it from being accidentally opening.
That said, at least some of them have put the HDD in the back now, instead of near the front (don't know if it's all). It could be that the magnetic closing was part of the decision to move them.
Hard drives have two extremely powerful neodymium magnets in them anyway, attached to the arm which moves back and forth. Millimetres from your platters. I have a hunch that a fairly steady magnetic field is probably fine, it's erratic/alternating magnetic fields which are likely to kill things.
That said, at least some of them have put the HDD in the back now, instead of near the front (don't know if it's all). It could be that the magnetic closing was part of the decision to move them.