qt - setText outside of paint events not ok?

In general you should not update Qt controls from non UI thread, only a small amount of things is allowed to do regarding a painting in non UI thread - http://doc.qt.io/qt-4.8/threads-modules.html

If you need to update UI from non UI thread - use signals and slots (QueuedConnection or BlockingQueuedConnection connections, though make sure to not create deadlock with BlockingQueuedConnection). Or if you don't want to create additional signals and slots for some easy update - use invokeMethod (it can even return value and if you use it with BlockingQueuedConnection connection type, your thread will wait until UI is updated).

And a general advice - if you have a possibility - make one call for bulk of updates to UI instead of few small calls.