[m21sniper]

Parallax shift is theoretically not an issue for a reflex sight.

Wiki on Reflex sights:

Reflex sights use refractive or reflective optical collimators to generate a collimated image of a luminous or reflective reticle. This collimated image is reflected off a dichroic mirror or beam splitter to allow the viewer to see the field of view and a reflection of the projected reticle (e.g. a red dot) simultaneously. If no magnification is utilized, this gives the viewer a theoretically parallax-free image of the reticle, superimposed over the field of view at infinity"

A reflex sight with no magnification can be held at any distance from the eye (see eye relief), and at almost any angle, without distorting the image of the target or reticle, and without causing the reticle to "move" relative to the target. But parallax compensation is not perfect, and depending on the sight's design, the range to the target, and the magnitude of angle at which it is looked into, aiming error can be non-trivial due to parallax.[2]

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It depends on the sight. Some are better than others. But generally speaking at close (ie gunfightin') range, with the weapon mounted to your shoulder even close to properly, it will not be an issue of any concern.

Use your cheek to stock weld to ensure that you maintain a good firing line down your optics.