It's probably better to store in native precision and then round on presentation. As an example, if you add `0.1` and `0.2` at the Python prompt, [you get `0.30000000000000004`, because 0.1 doesn't have a native representation in floating point](https://docs.python.org/2/tutorial/floatingpoint.html). Discretisation is ultimately up to the user: equal width frequency binning and stuff like *ChiMerge* map float intervals into integers, which avoids the problem.