Is there a Web server 'default' best practices?

I have always configured a web server to default to a certain domain that I'm pointing to it. I just setup a web server with a few domains on it and realized I should be using a default sites-available config... I think.

Is there a best practice for setting up a web server with multiple domains? Right now, I have a self signed cert for the default config, in case someone points to my server for some reason OR if there is something done outside the configs for each domain. Since the default config has no domain associated to it, I had to use a self signed cert. Also, right now, I just have default return a 404.

If it makes a difference, I'm running nginx.

Is there some docs out there on what is best to do here? Thanks for any input.