As with many spells from champions in League of Legends, it really depends.
Things that you should bear in mind whenever considering harassing the opposing carry with your volley (and really any spell from a champion while laning).
What is your mana pool?
For example, if you're level 6 and you are going to drop your mana below what your Crystal Arrow (ultimate) needs, you potentially could be forfeiting an opportunity to secure a kill before your mana pool regenerates to a level you need.
What is your health?
For example, if you're low enough on health that one burst from the opposing carry will kill you, making the attempt is putting you at risk. It seems obvious, but is easily overlooked. Of course, auto-attacking will also do this, but is always something you should consider when deciding how to harass the enemy.
How far pushed is your lane?
If you are half-way up the lane or more, applying volley will usually hit a number of creeps at once and push the lane. Which is fine, if that is what you want to do, but typically in lane you want to avoid pushing your lane too far and leaving you vulnerable to ganks from the enemy jungler. You can, however, hit the enemy with an auto-attack, as this will not apply damage to the enemy creeps, and so will not cause your lane to push forwards.
On the flip side, you want to look to hit the enemy with volley if you're pushed on to your tower, it will harass the enemy and 'push out' allowing you to farm the enemy creeps without your own turret taking them out before you can kill them. This increases the potential gold you can get. And if you can hit your opponent as well, you're killing two birds with one stone.
Where are you in relation to your/your opponent's creeps?
If you hit the opponent with an auto-attack and you are ahead of your creeps, you are opening yourself up to being hit by the opposing creeps. If there are enough of them nearby, and it takes you enough time to get safely behind your own creeps, you could take as much damage as you deal, essentially making your harassment meaningless. Volley has a longer range, and you may be able to find an angle to land your volley on the opponent without moving yourself into harms way.
The health of the enemy creeps
For beginning players, one thing I notice happens a lot, is that they will neglect to farm creeps if they think they can hit the enemy. If your enemy has enough sustain, they can get hit occasionally by you, but it not put them in any serious danger. If, however, you neglect to farm your creeps, you are surrendering gold. And the more gold you have as Ashe (and any carry), the more items you can get, and the more items you can get, the more of a badass you can be. If you hit the enemy and not kill a creep. And the enemy kills a creep and doesn't hit you, and does this enough times, they will eventually have more items than you, and can kill you 1v1, and be more effective at destroying your team in team fights.
One handy thing with volley, as mentioned before, is that if done at the right angle and timed correctly, you can hit your creeps AND your enemy. This way you can harass the enemy, and land the killing blow on the creep. Do this, and you achieve what you are looking for without surrendering any gold.
There are many other reasons why choosing volley or to auto-attack to harass is not a matter of yes/no, hopefully these tips will help you in making a correct decision and improve your Ashe play.