Do you have a problem with performance/load on your Tomcat server? Do it if you need to(performance, security, etc), but don't make things more complicated if you don't need to.
It used to be the standard to front Tomcat with an Apache server, but recent versions of Tomcat can(and often times are) used as both the HTTP Server and the Servlet Container.
Take a look at the Tomcat Connector FAQ for information on the subject.
Why should I integrate Apache with
Tomcat? (or not)
There are many reasons to integrate
Tomcat with Apache. And there are
reasons why it should not be done too.
Needless to say, everyone will
disagree with the opinions here. With
the performance of Tomcat 5 and 6,
performance reasons become harder to
justify.
...
Speed. Apache is faster at serving
static content than Tomcat. But unless
you have a high traffic site, this
point is useless. But in some
scenarios, tomcat can be faster than
Apache httpd. So benchmark YOUR site.
Tomcat can perform at httpd speeds
when using the proper connector (APR
with sendFile enabled). Speed should
not be considered a factor when
choosing between Apache httpd and
Tomcat