With the upcoming release, the DeCONZ light groups will expose an all_on attribute, it’d be great if we could have the same attribute also for the Hue groups.
Building on it, I think it would make a lot of sense to have a setting in Hass to define which state the user wants to use to handle the toggle of the groups. In my case I prefer to have all the lights turn on if I toggle a group that has just one light on. And have them turn off only when all of them are on. (but this is a separated feature request)