Looking at it from the standpoint of resistance, when the lever arm gets longer, the resistance gets greater. So, if you lean away from the side of the dumbbell there would be a shorter lever when your arm was perpendicular to your body.
To use a more extreme example, if you did the exercise while lying on your left side on the floor, there would be no resistance at perpendicular.
So, given a choice I would lean toward the side of the dumbbell.
With free weights, the resistance is only pointed straight downward. So when you stand upright with a dumbbell in your hand there is zero resistance on your side delt. As you start to raise your arm the resistance on your side delt starts to increase. The problem with that is that it is the opposite of your body's strength curve. You are strongest at the bottom part of the movement and much weaker when your arm is parallel to the ground. With a dumbbell, the resistance is at its lightest when you are in your strongest position and the resistance is at its greatest when you are in your weakest position.
Holding the dumbbell in your right hand as you start to lean to the left you start to increase the resistance in that bottom position. Taking it to the extreme if you lie on your left side comletely then all the resistance on your side delt is in the bottom position, your strongest, and no resistance at the top position. So, though not perfect, you have to play with the angle so that you can more optimally balance the resistance to your side delt. When you lean to your right, which I see most people do, you only make the situation worse.
This is what Arthur Jones tried to compensate for with his cam which stimulated variable resistance. In the case of his lateral machine the cam peaks, causing more resistance, at the bottom of the movement, then start to tamper off, gets "lighter" as you progress through the movement.
Another example is with squatting-type movements. You are weakest at the bottom, full squat position getting stronger, or more precisely in a more advantageous strength curve, as you start to extend and push through.
This is important because it's similar to the saying, "a chain is only as strong as its weakest link". Because your joint moves in a circular pattern and free weights are not rotary but only presents a straight downward resistance, you are only able to handle a resistance that you can handle when you are at your weakest. That's why you see so many people using literally tons of weight on the 45 degree leg press but you know that if they broke a 15 degree angle the weight would collapse on them.
That's where force reps and partials come into play. For instance, after doing a leg pressing movement, say, on the Hammer machine, after failure, I immediately position the set back so that my legs will only bend at about 20 degrees and increase the weight considerably and bang out those partials when your legs are in its strongest position.