Does the use of 就 and 便 vary by region? 就 and 便 are synonymous in many sentences:


  这儿（就、便）是我们的学校。
  我看了报纸（就、便）扔了。
  如果你去，我（就、便）不去了。


I believe that 便 is more formal than 就; I've seen it in writing but never heard it in speech. However, my Chinese experience is mostly limited to the south, and 
perhaps it's common in other regions.

Is 便 a common subsitute for 就 in certain parts of China? In places where 便 is common, can it be used everywhere that 就 is used? Are there sentences where 就 is appropriate but 便 is not?
As a native speaker, I certainly never encounter anyone from any part of the mainland who uses "便" in colloquial language frequently, if ever. I'd say "便" is basically a written equivalent of "就".