Hi, Andrew—A comma would be good after "line," since "ultimately" modifies the whole clause "that's the bottom line." If you wish not to use a comma, you could place "ultimately" after "is." With other placements, the comma is desireable:
- So, ultimately, I think that's the bottom line.
- So I think that, ultimately, that's the bottom line.
- So I think that's ultimately the bottom line.
- So I think that's the bottom line, ultimately.