tk, I do not think any of those three universities will have TAs engaged in instructional pursuits to a level worthy of concern. For example, at Michigan, only 3% of all undergraduate classes are taught by TAs. The vast majority of those will be introductory level writing, language and mathematics classes.
The Biology department does not usually use TA for teaching. Most of the work TAs do at Michigan, as their title would suggest, is assist faculty as lab or discussion leaders. I doubt UVa or BC use TAs more prominently than Michigan.
Also, I see nothing wrong with the concept of TAs teaching undergraduates. At most elite graduate schools, PhD students are as brilliant as faculty, and are probably more in touch with undergraduate students, making them more effective teachers.