TOEFL Score and Teaching Assistantship
Note that TA is normally only granted to a (usually) doctoral student. You need to be accepted into a program before you can be considered to be a TA and that will be determinative. 

The evaluation will be different at every university as we have no national system here. Also, TAs do different sorts of things in different fields and in different universities/departments. Some actually teach undergraduate courses and some simply grade papers. In between are those common cases where a TA will work a few times a week with a small group from a large lecture section. For most of these situations fairly good English skills are needed, but each department will do its own evaluation. But it is probably understanding and speaking ability, more than writing, that is the most important thing as a TA. 

Treat TOEFL as important but there may be other considerations that are treated as more important in a given situation.