Check out More on ambigious "should" by Mike Pope<firstname.lastname@example.org>. Here is an excerpt:
I was reading a thread on a computer forum, and someone asked this question:
Quote:Your password should contain at least 6 characters
If you’re going to require it; don’t say “should”, say “must”.
This set off an interesting discussion on the semantics of should in this context. I’ve written about…
The full article is available here.