When is it worth it to use Triggers and stored procedures/functions? As an example, lets say I have a forum application and for every topic that is created, I have a trigger that fires a database function creating a post.(lets assume we have a topics table and a post table)

I know these things can be quite circumstantial, but generally, do simple tasks like this belong in triggers/functions, or should they just be done in the application code?

As for updating views and replies I was thinking of offloading them to a queue, since the systems functionality doesn't really depend on them.
Triggers are problematic because they tend to get forgotten about.  I only ever use triggers for auditing purposes such as updating a "last updated datetime" column.  Stored procedures and functions can be used at any time since they are called directly by processes wishing to modify data -- as opposed to triggers that are fired as the result of data being changed.

If you are building queues to do some of the work, I suggest you also use queues to create your posts as well.