[Zeke]

"I think that having the sides and top level, plumb and square will make the rest of the door frame build/install much easier and better."

Always the best practice. You can cut the bottom of the door with a gap parallel to the floor and all the visuals look good.

And if it makes any difference, should anyone put wallpaper or vertical paneling on the walls, plumb and level is good. I know wallpaper is not in style so much, but back early in my career we saw a lot more of it. Out-of-plumb corners and casing made for one hell of a job.