Yeah I make mine light so that it is noticeable but not so obvious that someone has to chop it out to enjoy the image. some people watermark the entire image or right across the middle from corner to corner. Its all personal preference and it also lets people know that it is your image originally.

Even if you do not register your mark you have legal precedence. Since you have the original file as well as the original watermarked file then you can use that as evidence if someone were to use it in some way without your permission in say a publication, website, etc. As long as it retains that mark and is not used in publication they are within their rights. Once it is added without credit or payment they have overstepped their boundaries.

Hope that made sense