" I should follow this up with guidata, but I haven't identified a place where this actually matters."
It only matters if you are actually changing the data inside handles. Neither of your examples do this, so calling guidata at the end of those callbacks is not required. The only time it is required is when you actually change something inside the handles structure itself, e.g.:
handles.mydata = 1234
and you want that data available from within other callbacks in your GUI.
"What does guidata do?"
It stores whatever variable you give it in the parent figure of the handle that you give it. When using GUIDE the variable is handles and the parent figure is the GUI figure. Actually the variable does not have to be handles, but of course your GUI won't work if you replace handles with the string 'hello world', so storing another variable there is only of interest if you want to stop your GUI from working.
