What I had done was keeping just 1 gallon going from the dregs I liked as my "house" sour culture. When it was time to brew a full batch, I'd pitch a full pitch of my primary yeast for that batch (typically a Belgian or Saison strain, sometimes US-05 or 34/70), then add some of my "house culture" to it. I didn't really see the need for keeping a perpetual starter going for commercially available strains, since I could simply buy a fresh pack if needed.