The symmetry checks currently being used in PyCogent/QIIME are implemented this way and we haven't seen any users run into issues yet. Do you think I should add a tolerance parameter to the constructor that defaults to 0.0? This would be similar to [scipy's is_valid_dm](http://docs.scipy.org/doc/scipy/reference/generated/scipy.spatial.distance.is_valid_dm.html) function. Or do you have another idea of how to handle this?