In theory you should.

But in practice, you often have a problem with where you place the supports. When they're placed on an angled surface, they often let the airplane tilt when it reacts to the angle. The top of the airfoil is tapering in the opposite direction than the bottom of the airfoil everywhere except at the point of maximum thickness. So if you put the supports anywhere on the win other than the point of maximum airfoil thickness. those supports are going to be trying to hold to an angled surface.

What usually happens when you try to balance a low wing airplane from underneath is that you first have a problem with the underneath's angles. Then the CG is going to be way above where the supports support the structure. Try balancing a pencil in your hand, point down, and you see the problem. The lower the wing is on the low wing, the harder it is to get it to balance steadily.