Idealized models are a wonderful way to get a quick grasp of how an optical system might work. But be careful! Real life will never meet your ideal model.
Overconfidence in idealized optical models is a common pitfall, especially for those relatively new in optics. The ideal world our theories are based on is imaginary. Real rays of light have real widths. A real plane wave extends to infinity in all directions. Real lenses aren’t perfect optical elements that are free of aberration and focus light to a single point. And there’s no such thing as a perfect point source.
There’s nothing wrong with making estimations to make it possible to do your calculations— but it’s important to realize what those estimations are and be aware of the confidence intervals in which your figures work.
Avoiding overconfidence in idealized models will avoid nasty surprises when your optical system doesn’t perform as expected.