Just an app dev, not a geospatial expert, but reprojection always seemed like something a library should handle under the hood unless one has specific needs. I'm used to the ergonomics / moron-proofing of something like Postgis' `ST_Distance(geography point1, geography point2)` and it gives you the the right answer in meters. You can easily switch to spherical or Cartesian distances if you need distance calculations to go faster. `ST_Area(geography geog)` and it gives you the size of your shape in square meters wherever on the planet.
What the "right answer" is will vary widely on application and analyst. That's one reason there are so many coordinate ref systems. If you keep everything in 3857, you'll get answers in ~meters, but whether that's "right" depends on where and how large the distance or geom is and what your precision threshold is. So, really, everyone's needs are necessarily "specific."
