When I freelanced, since it was work for hire, I had no problem giving the client all the work that made up the project. They were paying for it, so project files, assets, renders, whatever, they can have it. 90% of the time they didn’t have either the plugins or expertise to update the work and would come back to me anyway.
Of course anything you have bought extra for a project, you probably legally can’t give them, so you should keep that in mine too.