In a lot of my APEX 3.x projects I used jQuery to extend the standard functionalities of APEX with some more interactive (client-side) components.
In jQuery there are very nice plug-ins, like a modal window, a grid, tooltips etc.
If you try to run your APEX 3.x application in APEX 4.0 you might find these jQuery components/plug-ins are not working anymore. The cause can be multiple:
- You might have used an older version of jQuery and the plug-ins you used with that, which are not compatible with the jQuery version in APEX (1.4)
- You load jQuery, but APEX loads their build-in jQuery too and things get mixed up
Before I loaded my files as part of my Page Template, but I'm not sure I would still do that.
If you switch from template you lose the references. It might be better for maintainability point of view to include it in a region on Page 0. It will have a small performance hit, as APEX will have to look for it on Page 0, but it makes maintainability easier.