Great question. I normally transfer copyright to all of the published files and source Articulate files after I have received final payment. I feel its something that can be discussed at the start of the project, and if the client wants the source files you can include them in the proposal as a deliverable. I also know designers who add a little bit extra cost to the project seeing how it is another deliverable. Clients don't normally own a copy of the Articulate software, so source files become useless for them.
That being said, if you want to use their work as part of a portfolio that is going to be displayed publicly for you to market your services I recommend getting the client's permission. Some clients have even told me it's ok for me to use their project as a sample of my work on the condition I remove their company logo and any trademarks from said work. Easy enough, but it really depends on the client's sensitivities.
As for the above, I have a clause in our proposals under the heading "Copyright" that goes something like this:
When we receive final payment, copyright is automatically assigned as follows:
You own the published course in .html format, written content, images and any other visual elements that we created for you.
You also maintain ownership of any written content, multimedia assets and other data you provided in order for us to complete the project.
If the client wishes to also own the course in the Articulate Storyline format, you can simply add it in there as well. One thing about me is I try and limit the legalese from all agreements as a matter of principle.
Hope this helps,