guidata() updates the "master" copy of the handles structure. Any routine which has its own local copy needs to use guidata() to fetch the updated version. If you have a routine which does a read / modify / write cycle then you need to be sure that nothing else has updated the master copy between the time you did the read and the write. For the most part the defaults about Queuing Action and related properties work out, as traditionally one graphics routine was not permitted to interrupt another. But timers can interrupt graphics routines, and listeners might play a role (I do not recall what the documentation says about when listeners are called), and of course any routine you call yourself might update the structures.
This is the reason why it is necessary to design in "interface contracts". If you have interrupt routines or routines that you call explicitly, you need to know whether they have permission to change things "underneath you", and in any case where you cannot prove that nothing has changed you need to re-fetch the data or you need to use a semaphore or barrier system to ensure that you have exclusive access to the mutable resource during the critical period.
