My friend Ben Claremont of Life in 360 shares an important technique for using selfie sticks for 360 photos and videos!
Here is the video:
For clarification, there is an exception to this general rule. Some cameras don't have horizon correction. For some cameras, the horizon correction works for photos but not videos, and for other cameras, it is the reverse. If your camera has no horizon correction, then you have to make a choice -- either you correct the horizon manually afterward using 360 editing software, or you use the camera upright, with the selfie stick visible.
Personally, I tip the balance in favor of correcting the horizon manually when I'm taking a photo. There are several apps for correcting the horizon such as Theta Converter (for Android), Hugin, or Flexify.
For spherical videos (intended for viewing via a headset or a smartphone VR window), there are software that can correct the horizon such as Autopano Video, but they can be pricey. (Incidentally, the Insta360 Air will include horizon correction for videos in its app, which is one of the reasons I like it.) Therefore, for spherical videos with cameras that don't have horizon correction, I tend to keep the camera upright even though this will result in a visible selfie stick.
You can subscribe to Life in 360 on YouTube, where Ben shares many valuable information and techniques for 360 photos and videos.