On chakras hours specific chakras are more active and it is more easy to feel them and to work upon it and reap more benefits from meditating on it.
But this is not mean other chakras are closed and unreachable on that time.
In complete chakra meditation if you do it on all your chakras at once it has no big difference, just that chakra upon which hour is at the current time will be mostly affected and mostly easily worked upon because it is more active then than others.
But If you want to work more on specific chakra, like opening and etc., of course if you have possibility for getting most out of meditation, it is better to do it on planetary hour.