When should I use the multi-db plugins? Now or later?

Hi There,
I'm planning to build a platform using WP network. Of course in the beginning, there won't be a lot of users so one database is good enough. If, later on, I have a lot of subsites, then it seems to me the plugin Multi-DB will come in place. My question is: When should I use the multi-db plugins? Can it be used later (won't impact existing Data)

Thanks
Leo