If components have been derived and altered such that they no longer respond to actions in the same way (i.e. rewriting the single-click event for a textbox, non-standard responses to certain key combinations, etc.), they may no longer be testable out of the box.
It is therefore recommended for the purposes of testing – and also for the sake of conformance to usability standards – that the toolkit conventions are held to.
There may, of course, be situations where exceptions are necessary. For this reason, there is an API via which testing classes can be implemented so that even changed components can be tested. See the Developer Manual for more information.