TL;DR version in the picture.
A few months ago, someone made me notice that I had placed Twilight's cutie mark on one of my vector on the wrong side, so I checked a few other references, and he was right. I had pretty much always used a pre-made cutie marks, adn never really payed attention to its position until his comment; well at least for Twilight. I noticed Rainbow Dash's way before that, but hers is really obvious.
Anyway, after his comment I started paying more attention about it. But for some reason it didn't occur to me to check for the other 4. I pretty much assumed they were symetrical.
A week ago, my train of thoughts lead me to this particular comment, and I ended up thinking that they were in fact not symetrical. So I decided to check it.
I checked several episodes from the three seasons, and they all match. They're all positionned like in the picture above. Fluttershy and Applejack's are very subtle, but it's still not symetrical.
I quickly went through several folders, and a lot of vectors got the cutie marks backward (some of mine included), so hopefully this guide here will prove to be useful.
Cutie marks by