With scenic still images, I place it in the lower corner so it does not interfere with the image...for products, I plaster it right in the middle of everything....

HD videos are the scenic thing typically, so I about the same as what the Discovery Channel, History Channel, NatGeo etc do, all in the lower corner.

Even with plastering things right in the middle, I watched a tutorial using Photoshop 5 where the speaker took an image he shot of a hair ghost pipefish, talked about how deleting it from the image would take hours, then proceeded to remove in in 30 seconds, and worse, he used some tool that filled in the background behind where the hairy ghost pipefish used to be to make it look like a real overgrown reef just like adjacent background areas of the original photo.

That makes it REALLY easy for image thieves to delete a watermark right in the middle of everything....

But I know what you are saying....

iWatermark has an embossed 3D tool so it uses the image itself rather than blotting it out....

Photoshop has a tool and plug in vendor that allows you to embed a invisible watermark then trace it wherever it goes....