Sure, it is your call. As said, this is rather an enhancement.

Still, if I were to decide, I would chose:
1. not to break the API <=> raise IndexError instead of ValueError in case hi is invalid.
2. to protect against illegal values: as said, if hi < 0 bisect_* always returns 0 (whatever the searched value).
3. I would implement a single _range_check(_len, lo, hi) method to do this logic (DRY).

That being said, from my point of view this ticket can be closed.