yes I do everytime I upload something. People are stealning so much online you really need to. They even use personal photos. For eg. photo of shomoney holding his google check is used for advetising a scammers website without a permission so be carefull and always add watermarks
I don't now, but as i'm starting to do more I think I should. I think it is a good idea to prevent people from ripping you off and claiming your work as their own. Just make sure you save a copy on your hard drive WITHOUT the watermark, and you'll have it in case you want to make prints or submit it professionally.
I do water mark my photographs. I am not comfortable with online tools where one has to upload the snaps to watermark. I am not comfortable with that concept. Thankfully contenta converter that I use for conversion and processing also has this feature and has the feature to select the transparency level. I don't think water mark can ruin your snap. It is one of the best ways to protect your snap