For me it depends on the image & the customer. When I deliver a CD of 600px proof images, they have my logo on the picture usually in one of the corners. The customer can use them for emails, blog postings, & stuff like that, and I let them know they will have my logo.

If they buy hi-res jpgs from me that they will be printing, then I leave my logo off. I make sure that that info is passed onto the customer before we shoot so we are both on the same page.