Some people say truth is not absolute but cultural and dependent on context. I think this is wrong. "Truth" should mean that which is the case. On the other hand I'd like to know a term for a proposition that, if agreed upon and treated as true, has value within a certain group or context although not in the absolute.
Are you talking about (cultural) relativism?
A theory, especially in ethics or aesthetics, that conceptions of truth and moral values are not absolute but are relative to the persons or groups holding them.
A proposition that is culturally relative can be referred to as a "cultural truth" (as in the title of your question), or a "cultural truth claim".
The simple word belief might serve your purpose. I was also going to suggest the sociological term more (pronounced MOH.ray), but I can't even find it. I thought there a term spelled m.o.r.e, but it might have more to do with cultural values than beliefs. Maybe I need to ask a question about that one. ;)