A guid would certainly be unique enough, but the idea is really to use a consistent identifier, if you have more than one instance of the control on a page by having the same type and id string it will only add the script once. If you needed it to load the script separate for each instance then you would need different ids in each instance. Typically the id can be some simple string like "myslideshow" or whatever, a guid works but is a little unconventional.
Best,
Joe