I would go with what larger established tools like Ember aim for:
On the other hand, while there is enormous support for dropping IE9 support as well, a number of people still rely on support for IE9, and the benefits of dropping IE9 in Ember 2.0 are not as strong.
After reviewing discussion on the RFC proposal, many in-person conversations with Ember users in large companies, and reviewing the private data sent to us via email, we have decided that Ember 2.0 will support IE9+.
How big of a deal will it be to keep support for IE9?