Aiming the flash more closely along the umbrella’s axis can help the umbrella work a bit more efficiently and may reduce spill outside the modifier. It also lights the umbrella more evenly, which can matter if the flash is noticeably off-center.

In practice, if your flash is only slightly above the axis, the difference is usually small and may not be noticeable in normal photos. The main visible issue from an off-center flash is that the umbrella can be lit asymmetrically, which may show up as uneven or asymmetric catchlights in your subject’s eyes.

So: centering the flash is generally a good setup habit, but don’t expect a dramatic before-and-after change unless the flash is significantly misaligned.