In most cases involving control events, one can abandon events without disposing them and not run into trouble, because the event publisher and subscriber will generally go out of scope at about the same time. In some ways, this is unfortunate, since if event cleanup were considered necessary for correctness, there would probably be language support for it, and any decently-written code would clean up events as a matter of course.
The problem is that anything keeps an abandoned event publisher from being garbage-collected will also prevent any of its subscribers from being garbage-collected; if those subscribers publish any events of their own, all of their subscribers will likewise be needlessly sheltered from garbage-collection, etc. Suppose, for example, that one has a program that's working beautifully, but one adds an event to let a form know when another form is opened or closed, so that each form can have a "Windows" menu listing other open forms. Nice feature. If, however, a form is disposed without unsubscribing from the event providing such notifications, such failure may prevent the form, or any of its controls, nor any other objects to which those things hold references, from ever being collected. A significant memory leak, though one which will likely only cause problems if multiple documents are opened and closed during the lifetime of the program, and which may thus not be discovered except by the end user.
My recommendation would be to use the form's Disposed event to handle event cleanup. One could change the Dispose code generated by the designer, and the designer would probably leave those changes alone, but given that the Disposed event exists, I would think it cleaner to avoid any tinkering with the designer-generated files.
