Yes and no. umask is something that is sometimes good to change. Generally, it is best to "default deny," as in default to the least privileges with most things. You will likely find that if you change the default umask and are not particularly aware of permissions that you may encounter situations where the default permissions prevent things from working.
For example, a PERL module install via CPAN that doesn't set the permissions public readable as part of the install and then later a Web application is unable to read it.
Really, the main benefit of hardening permissions will be seen with a multi-user system. Nevertheless, it is important to be particularly aware of anything that's easily accessible within an Internet accessible daemon.
For hardening, it is often better to take a look at anything SUID or public writable. I find myself verifying permissions and tweaking them to least necessary privileges just as I go about my day. When I change umask it is often for a particular purpose such as with a particular set of users or when files are created programatically.
If you're looking for some "best practices" for your server and general hardening techniques, take a look at Bastille. If I recall correctly, there was something in there that changed the default umask.
Bastille
