If use of Multi-DB reversible? Is it a bad idea for some?

I'm redoing my website and hope that it will be a very big site someday (using multisite). However, it's not "big" yet. Should I use multi-db yet? Or should I wait? If I do use it and set it for lots of databases, what happens if I turn off the plugin? Is this a permanent type of change I'm making to the database structure? When should I and should I NOT use multi-db?