Simply trying to UV map a hemisphere. Using a UVTexture SOP with Polar projection.
Doesn't seem right to me.

Is there a best practice when UV mapping sphere's that also avoids seams?

A sphere when mapped using polar coordinates will have a seam along a line of longitude.

You could try to use the GameDev Toolkit AutoUV node and see if you get better results. You might or might not be pleased with the solutions that AutoUV gives you, but ultimately for correctly mapping a texture on a sphere in such a way that it doesn't distort at the poles, the texture itself needs to be distorted to compensate for the squished UV layout. I think mathematically wrapping a 3d sphere as a 2d plane has been a difficult problem to solve, just look at world maps that still aren't quite correct.

I think mathematically wrapping a 3d sphere as a 2d plane has been a difficult problem to solve, just look at world maps that still aren't quite correct.

This is true as there can never be a ‘true’ 2D representation of a 3D object.

Although certain projections can preserve aspects of the 3D object at the expense of others.

In the typical maps that you see of the world in which the land masses grow proportionly larger in size closer to the poles, e.g. the ones in which Greenland is huge;

What your seeing in such a cases is called a Mercator projection or varient thereof. In such cases direction/bearing is preserved at the expense of relative size.

Since the primary purpose of maps historically has been for navigating, is the reason why we typically see Mercator projections for the most part(dated publications) as this form of projection has been the most used and available.

For other projections when on wants to see the ‘correct’ sizes like Mollweide projections, one loses direction/bearing information, but can see the ‘correct’ relative land mass sizes.

So when it comes to uvs, although we don't have to think of the types of innumerable projection types or ways to do so, we can visually through like with the uvquickshade node determine our best desired approach/s.

How about a hemisphere? I saw a video where they cut the sphere in half and map each separately. But even still, I'm getting distortion along the edges. My goal is to map video to a sphere so can't have any uv breaks. I'm thinking I could spend hours trying to uv edit. So far, vertices/polar is the best and will just have to live with distortion at the poles.

I changed my sphere from polygon_mesh to just mesh and it fixed my seem issue I was having. Hope it helps someone else too.
See attached images.