Almost everyone does it wrong, they pass the value to a state, this is very similar to how Angular and $scope worked in two-way-binded. Every letter typed does an update. why not just get the values on submit?
With web, it's very common to do this, they pass the form submit and get the value based on the element's reference. for React Native however, we setup two-way binding with the component state. But WAIT, there is another way!
Using TextInput vs Input from React-native-elements, getting the value of inputs, I advise not updating state on every keypress, it feels way overkill, we should be able to just grab these values on the form submit using the _lastNativeText (https://github.com/facebook/react-native/issues/511)
Everyone uses the state for this, on Web, they use the DOM, simple