Honestly, allowing it at will won't break the game. 7, 10, 14 and finally 17 damage for an action, even in a small area, when the DC is not going to be your highest DC possible.... just fine. I tend not to change the rules of the game, but I did give one dragonborn PC a magic item that allowed him to use his breath weapon as a bonus action, and to recharge it on a roll of 6 (similar to a dragon). The PC was far from overpowered before finding the item, and ended up not using it as often as he could.