When is it appropriate to use watermarks?

[stitchedlamb.livejournal.com]

I hope this post is ok, I just wanted some opinions from people who are more experienced at taking precautions to protect their work.

As I'm sure most of you know, DeviantArt has implemented a watermark system that you can choose to use every time you upload a piece, which shows up on full view. I'm aware it's not a foolproof method, but it's something, yes?

I just wanted to know if it's a good idea to use watermarks in general, or if you should only resort to them if you've encountered theft before? I know most people don't like looking at them, and since I've never had a problem with theft, I don't know if I should start using them or not.

Basically, is it ok to start taking precautions now, or only use them when I've already had my work stolen? In all honesty, I guess I'm worried about looking arrogant and assuming someone would steal my stuff. I'm not all that concerned about it since I'm not well known, but since the option is there, I wanted to know what everyone thought.

Thanks guys!