General .htaccess question

I think I know the answer to this, but want some confirmation from someone a bit more knowledgable about Apache and .htaccess files. My webhost has the default 404 page set to 404.html. For the sake of consistancy on a number of levels, I prefer a PHP file for this (404.php). So to do this, I created a basic .htaccess file for this very purpose and it sits in the root dir. Now, does this .htaccess file get read for EVERY resource request on my site - even when the page is found? Aside from a META redirect from 404.html to 404.php, is there a less server-intensive way to set my 404 in Apache when I don't have root access? Thanks -M