Passwords won't keep out a determined intruder, but they will keep out the casual intruder or troublemaker.
Still, if you're among the truly paranoid, consider creating a separate, dummy account (see Project 16) and fill it with seemingly important files so that if someone breaks into that account he or she won't know to look elsewhere.
You can turn on the automatic login feature to load this dummy account any time someone restarts your Macintosh in an attempt to gain access. A dummy account (also called a honeypot), can mislead intruders. If you create several accounts and password protect them all, an intruder would need to know each account password, but he or she would also need to know which account contains your crucial files. This convoluted setup may make take some time to create and organize, but if you have important data that you need to keep away from prying eyes, it can keep potential intruders from invading your privacy.