Hi @marina747,
301 redirects are not mandatory: Phoenix Media Rename will update all references in your posts (all kinds of post) to the renamed files, you need 301 redirections only if you have links to your files external to your site (on other sites, social network or other sources).
If you remove (disable or uninstall) Phoenix Media Rename, your posts will continue to show or link your files, to create 301 redirections you also have to install and activate the free plugin Redirection, this plugin will manage all redirections, but only if it is active.
Let me know if you need other information.
C.