Go Master Ball!

I don't use it normally. I try and catch everything in Heal Balls because they are my favorite design wise. The Master Ball is really ugly to me and I don't want my babies living in that thing. I'll use it to catch a shiny though that I don't wanna risk losing.

I tend to sit on it forever, as well, but the only reason you *truly* need to use it is if you come across a shiny Abra or Graveler. Anything else you can throw Ultra/Dusk/Quick balls at.
(I catch all my roamers using Quick balls anymore.)