No - the "if" statement checks if you have enabled the plugin. It has nothing to do with the appearance of "No related posts". This just means that if you have no related posts, it will still show the "Related Posts" heading, followed by "No related posts". The "if" statement is good practice because usually when you perform a Wordpress upgrade, it is safer to disable all plugins before upgrading. And it means that you can leave the code in your template while the plugin is disabled, without it stuffing up your site.
Ah ok, I'm not much of a php coder. Besides, I don't think it'll matter that people can see the "no related posts" thing because over time I'll have enough posts that there will be a much smaller chance of that message showing up.