Pre-installation questions…

Hi there. My pre-installation question is pretty simple. If I install and activate MMR, and my site breaks, will deactivation return my site to how it was (until I sort what broke etc)? Another way to put it is will this plugin jigger up my css, js, or functions.php or anything else or is it a ‘turn on, turn-off to restore site to original’ kind of plugin?

Rationale for my question: I am working with a live (with high-traffic) site, without the advantages of a development area or programming knowledge. I call myself a reluctant webmaster… (volunteer work for not-for-profit org) I can not afford for the site to go down and then stay down while I try to find what went wrong. If it goes down, I just need to be able to restore it to its pre-plugin state without jumping through hoops 🙂