I just keep my head down or turn to the side. It's more because I don't like how I look in my pictures. The lighting is terrible and I'm usually pretty disheveled and tired.
My personal philosophy is that if you want anonymity or if you're concerned about stalkers, don't post pictures. Just chopping your head isn't going to prevent people from knowing who you are. They'll know your body and more importantly, they'll know your clothes! Not to mention you will assuredly leak some personal/private details about yourself at some point, like what city you live in or what your profession is...and in many cases you might even be using your real name as a username. Masking your head or face isn't really going to help.
I have done a lot of thinking about this and come to realize that the odds of me having a stalker or being "caught" by someone I know are pretty minuscule...but I don't want to increase the odds by posting daily WiW's or anything. I only post when I have questions or want to confirm something new that I buy.
