The eye piece adjustment just focuses the reticle. If your prescription changes then you would probably want to change but it wouldn't be a day by day thing.
Setting it is done on max power and parallax and with the scope pointed towards the sky or a white wall etc. Nothing that would have you looking at anything but the reticle. Also when doing it only look at it for a second or two and then shut the eye or look away as the eye will adjust and the setting will be off.