Re: light scope/ no parallax
I would tend to agree with RockZ, if you have an adjustable cheekpiece so you get a really consistent cheekweld then you can get by with out parallax adjustment.
The other thing you can do to answer your own question is to take a scoped rifle and set it up real steady and adjust the parallax to 100 yards. Then move your eye/head to the right and left and see how much being off center causes the crosshair location to shift. It will probably be about 1 MOA at max and for an average bad head position it will be 0.5 MOA and for a reasonable but sloppy head position it will be about 0.25. All of those numbers are plus and minus so you have to double them.
There are other factors to consider such as the magnification etc but you can get a general idea by doing what I said. Then you can decide what the size of your target animal is and see if the error is acceptable.
The Smokin Fur Rifle Club