It is ok to leave them at level one .when you have enough gold and allied hero pieces ,you can consider level them up .Because it will also contribute to squad members .

If you want to activate your Ally Power and boost your whole squad BR and stats, you should level up your allies.  Every time your level all your allies by 10 levels you activate the Ally Power. Take note too that u only need to level them up and no need to augment those heroes as augmenting for now are useless for our allies.