There seems to be a bit of variation in the vertical placement of belly-buttons, but I’ve never seen one located say, two inches to the left of center. Does it happen ? How common are off-center belly-buttons ?
IANABiologist. Organisms which are symmetrical are so from a very early stage of development. Whatever has to go wrong for the belly button to end up off-centered would also affect many other things and likely isn’t healthy for an embryo. On the other hand, vertical variations would occur when the development has already branched further, so it’s more likely for them to be benign (dunno by how much though).
For example, I doubt that you can get far with all your organs smaller on one side, but some people are living proof that you can survive if your brain is smaller than your gonads. 