Depending on the retainer design there may or may not be a reduction of the balls in the bearing. Though I agree that they serve mainly as an assembly convenience, there's no evidence that using one degrades performance in any way.
If you wish, there's no reason not to discard retainers, but in most cases no reason to do so. If you keep the retainer and aren't sure of it's orientation you can assemble the axle bearing and cup dry in your palm and observe which will make things obvious. Or you can use a simple test. Lay a pencil at an angle against the inside and outside of the retainer simulating the face of the cone and cup, the orientation that clears the retainer is correct.
To the OP- Looking at the specific retainers in the photo the closed side (counter-intuitively) nestles into the cup and the spindle enters the open side. Exactly opposite to your Redux post.
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