I took a picture of myself simply using my phone's rear camera selfie style. Obviously my face is flipped. So should I post it as is, or should I flip it first since everyone else sees my face that way?
Your face is not flipped when you take a selfie. The preview is flipped to make it easier to use, but the image captured is not on any phone I've seen. Both the front and rear facing cameras work just like normal cameras. If someone was looking at you taking the photo and held it up in front of you, what they see in the photo would match what they see in real life.
If you want to flip it for some reason, you are welcome to do so. There isn't any rule against reversing an image and it is sometimes done for different artistic reasons, but it's entirely your call.
If you are just worried about it being backwards because you had the display facing you when the camera was facing you. That simply isn't how it works, so you don't have to worry about accounting for it.
Everyone sees your face the same as the camera does, YOU are the only one who sees it backwards when you look in the mirror. You have a perception of the way you look based on the way you see it in mirror and the way your brain thinks you look. Your perception of how you look is very different than how you actually look or how you are seen by others.
If you want to post photos that represent how you actually look then post the photos as they were taken. If you want to express yourself as something different than how people see you or how you want to be seen, then do that. But you should understand the difference.