As I’m sure you know, watermarking a photoset is usually automated. And in a batch of 100 pics, some important bits may be obscured in a pic or two. But I don’t look at it as such a negative thing though. Maybe the viewer will be intrigued enough to actually visit the site… heck… maybe even join the site to see more.
Generally though, it’s best to keep the watermark tight to a corner so the chances are you won’t often mess up a photo. But of course that does make it easier for jerks to crop off the waternark.