Not sure if you realized but the damage for volley/arrow rain is hugely dependent on strength. If you have little or no STR, volley will not hit enough to mean a difference.
Volley is also NOT combo-able so no one uses volley for training. And in quests, despite some guides saying volley is useful for some quests, this skill is largely useless. For the few quests where volley means something, you can easily compensate using your normal skills. When I first got volley, it was fun. I went shooting around but got bored soon enough. And not long after, I pretty much ignored the skill altogether.
You said you use bow for training - but that is only in the beginning when your character is new. Hold a blessed bow and shoot till you get bored and decide to get EC or do manual debuff (bursting) for better exp.
The Goddess Skill (G.S.) is something that you absolutely have to get though, especially if you are water or fire element. If you hope to level in a way that actually sees your exp move, you will likely start G.S. training as soon as you get more "pro". And unless you are wind element, you need the G.S.
When training, goddess skills also work as debuff. For quests, G.S. can be very useful too.
In the beginning, I can understand that stars seem so hard to get. But as you move up the levels, they come easier and easier. You can easily accumulate 30, 40 or more stars if you faithfully go exploring and do all the quests. You do not even need to be super pro to get them.
So to conclude, I would suggest you use your stars for volley only if you simply have too many stars or just want to have fun and check out the skill firsthand. Only warriors who add to STR should get volley but if you are a warrior, your skills would inflict much more damage than volley so you won't likely use a bow.
If you have limited stars, your priority should be the goddess skill. Then again, if you are not able to handle the moderate difficulty quests to get 10 or 20 stars, then I suggest you spend some time grinding your character. Just use the normal remote from Grandma at the cooking pot in Welling and train your pet (e.g. Elin) and yourself to a decent level.
After that, get a team and do some quests. Soon, you will find that you have so many stars that you have nowhere to spend on! :)