How should I think about sunlight for the purposes of analyzing the angles it makes w/ earth?
This is a first-order approximation, and works well for simple models of irradiation of the atmosphere or the Earth's surface.

But, a simple experiment with a pinhole camera or a single lens will show that this model doesn't work in imaging situations.  The sun is an "extended object," meaning it is not a point source, so the image you get is not what you would expect from a point source.  Worse, even if the sun were a point source, the distance to the geometric focal point is different very slightly from the distance if all the sun's rays were perfectly parallel.  See the Lensmaker's Formula,  $ \frac{1}{f} = \frac{1}{p} + \frac{1}{q} $ where p and q are the distances from the lens to the source and the image respectively.   (I'll warn that for a reasonable lens, say focal length $ f = 10 m$, the difference between focal location for rays that are parallel and rays from a point source $150\space million\space km$ away is ridiculously small) 

Similarly, if you want to model the umbra and penumbra during a solar or lunar eclipse, you'll need to take the physical extent of the sun into consideration.