Obviously, when creating a language/alphabet you can design it any way you want. Clearly flipped characters are realistic, so I would ask yourself - what essence are you going for? Are you going for an essence of practicality and readability? Simplicity? Elegance? Complicated and cryptic? Are you dealing with a language that's been in development for a long time, or a more basic alphabet that was designed to be quick and easy for everyday use?
Since flipped characters are more difficult to read - but they're also simpler from a design standpoint because it requires you to have less character shapes - they have a certain essence. Not one of practicality and readability, but perhaps one of complicated and cryptic because precision is needed. Such an alphabet might be impossible to read if scrawled out quickly, but maybe that's what you want. And then again, if you're going for very simple shapes and an "organic," almost mathematical or "tick mark," look for your alphabet, you might want to work with just a few basic shapes and rotate them.
Just some ramblings from someone who's designed one alphabet.
