If you allow your co-workers to friend you without taking privacy precautions on Facebook, they might be able to see, for example, who your other friends are, how often you post or update your status, comments you make on touchy topics such as sex, politics, and religion, and even your public answers to quizzes, such as "What 'Sex and the City' character are you?"
Granted, you can curtail what they view through Facebook's privacy settings, but organizing who sees what can be time consuming, as well as stressful as you try to remember who's seeing which postings. It can also be a social-media faux pas if you're partially shutting out some co-workers while allowing others full access to the information you post online.
So if you want to maintain peace in the office along with your peace of mind, your best bet is to refuse these friend requests diplomatically. You don't want to cause friction by simply ignoring the requests, or turning them down and making it seem as if you're unfriendly or, worse, hiding something. Rather, explain to all co-workers that you prefer to keep your home and work lives separate. Tell them you feel this is the fairest and least stressful way to handle these requests. Even if they balk, just remember that the only person with a right to your privacy is you.