Two newbie multi-db questions

First, when should I consider this? I'm guessing that since I expect to have less than 10 blogs, I don't need multi-db, but is that true? Would I be better off starting off from scratch using multi-db? My tendency is to keep the number of not-needed-now plugins down, but you guys will know better than me about this.

Also, if I have 10 blogs now, but I expect that, say, in a year I might open registration and go to 100 or 500 blogs, should I set up with multi-db now so everything's consistent, or wait until I actually have a big ol' pile of blogs?