I'd say if you want those features to work then you should test them. Consider, for instance, if the Rails implementation changes in the future. A failing unit test would be a great way to immediately let you know "Hey! The behavior of my app has changed!"
That said, only test the cases you care about for your application—no need to duplicate the entire Rails test suite. Definitely have a test for your "happy" cases where the input should be considered valid. If you think it's possible/likely that a decimal or string might come through, then test what would happen in those cases as well. If you find yourself doing this in other places, then make yourself some shared helper methods like check_doesnt_allow_decimals
, etc.