I think it can be either or both. For me I find a cut shot with a lot of distance between the cue object ball to be an issue. I look at the object ball but when I am on the cue ball lining up I seem to lose the perspective of where that spot was. I actually have found success in lining up the object ball to the pocket and pick a spot on the opposite rail on the diamonds. Then when I am at the cue ball I can draw a line from that spot and correct myself if need be. HMMM, maybe I should get an aim system as a back up, oh that is another story.
I do some instruction with some of the lower level players on my team. The most insidious issue with almost every player is not hitting exactly center on the ball. When they are down on the shot they will tell me they are pointing right at the center and they are not. Of course they insist I am wrong and they are right. I mean how could it be possible that they can't do something as simple as point the stick at the middle of the ball. Of course the true test is to take a stripe ball with the stripe standing up shoot it down the length of the table and bring it back to your stick, the stripe should not wobble. The interesting thing here is that what I told them about hitting the center becomes obvious but is almost always to one side. So, many players really struggle to hit center ball and I have had higher level players with this same error.
I believe if you have stroking errors you will be putting unintentional english on the ball and throwing the object ball left or right. This can be really deblitating as it will effect the players ability to develope shot memory. So, the value of a good straight stroke is undeniable.
However, as I said earlier I do have aiming issues at times. Want to check aim versus stroke? Put the cue on the head string put the object ball various places below the foot string and try a bunch of those shots.
Want to check for unintended english? Place the object ball on the head string. Using a stripe as the cue ball place that two diamonds behind the object ball. Shooting a straight in stun or stop shot the length of the table to the opposite corner did the stripe stay standing up? I will bet many a player reading this will struggle with this one thing. Even if you if you consiously correct this unintended english will creep back into your game.
Just rambling while drinking coffee slurp slurp