Psst! Center those horizons in your 2:1-aspect ratio equirectangular 360 photos, and your horizon will also be centered (and straight) in interactive viewers.
I take a lot of aerial drone panoramas, and I always have to stretch the sky (Content-Aware Scale and Fill are magic!) and paint a bit at the very top to get my horizons centered. The sky at the very top needs to wrap around the left/right image border seamlessly, and I get decent results by blending the sky at the top with a band of a single color (often, either blue or almost-white).
Here's the aerial 360 pano you see in the example above: