There was a thread in the Google group in April 2009 entitled "Should a Product Variation inherit tax attributes? " (Resolution: yes.)
This is a special case of a more general question, now that Satchmo has custom product attributes. I suggest that, by default, a product variation should inherit the custom attributes of its parent.
My application happens to be the one that the Satchmo documentation uses as an example for custom attributes: seling books. In this application, the variations are formats and bindings (e.g., hardcover, softcover, e-book). Here, some attributes should be inherited (e.g., "author") and some should not (e.g., "ISBN").
My first thought was that attributes should be markable on the admin page as to whether they were inherited or not. But I think it would be simpler for all attributes to be inherited by default. Then an administrator could fill in different values for attributes of variations as desired, which the administrator would need to do anyway if the attribute was not supposed to be inherited.