Rescaling DB with Multi-DB plugin

Hi

I have basic question about Multi-DB installation.

Is it better to think ahead and install Multi-DB before the growth of the site or is it better to wait to get 10000 users (if ever) and then try to install it and reinstall all the plugins like Supporter, Support System etc, that needs slight modification id Multi-DB is installed?.

Is it even possible?

Regards,

Peter