It can be practice, however, usually that discussion is exchanged prior to the start of the project. Usually, uninformed clients will ask; only because they have no idea on how the game works. What I tell clients is that I have no problem handing them over the editable work, however, at a charge because I'm handing them a template where they or another designer can make changes and updates to the work, thus having no need for my services. I tell them it's equal to a chef handing over his reciepe; surely the reciepe holds value over the initial meal. With the meal I have you coming back...selling you the reciepe it's all yours for a small token ($100 to $400 or more).