Yes, go ahead and split up as appropriate.
Things to keep in mind:
1) MATLAB can only locate the first function in a file from outside of the file, unless that first function specifically makes handles to the other functions available. For example, you could code
function callbacks = GUIcallbacks
callbacks = struct('Pushbutton1', @push1Callback, 'Freds_listbox', @fredlistCB);
end
function push1Callback(hObject, evt, handles)
...
end
function fredlistCB(hObject, evt, handles);
...
end
Then the main routine could call GUIcallbacks to get back a structure of function handles, and could set() the appropriate graphic object property to the appropriate handle.
2) Even if you have everything in one file, if you code callbacks as strings instead of as function handles, then those strings are always evaluated in the base workspace. The base workspace will never be able to find a function that is not the first function in the file (not unless you use a mechanism such as in #1)
3) Using one file per routine referenced from outside the file works fine... unless, that is, you want to start using nested functions.
