In our group, 'sensei' is a teaching title, the first one, as well as a generic term for "teacher" (or rather 'born first'). We teach that one does not generally become a true sensei until sandan...though for practicality reasons we really don't hold to that these days. For us any yudansha that is teaching can be called 'sensei,' but those who do not teach regularly are not necessarily promoted beyond sandan or yondan as there isn't much of a reason at that point.
Personally, I only like calling my personal teachers 'sensei' as it holds special meaning to me. Also, I feel uncomfortable when others address me as sensei.