That depends if you have some kind of "mangled worlds" hypothesis (in short, an hypothesis that worlds with a too low probability will be unstable and collapse due to contamination from "nearby" worlds).

As long as we don't know where the Born rule comes from in MWI, it's hard to say if all worlds are "real" and how much "real" they are, or if there is some kind of boundary below which the world isn't "real" for practical purpose (like, not stable enough to allow a consciousness to exist in it).