But unlike other shows, I don't really feel any "negative" rivalry in my shipping. Personally, I'm rooting for Miyuki for a long list of reasons, but that doesn't mean that I wouldn't like Mayumi or Mari, too.
Example: One of my fiercest shipping contests was in Hayate no Gotoku at the end of the Greece arc. I was and still am a devoted Hina shipper, and while I was noncommital-neutral on Nagi, I fiercely resented Athena (and, to a lesser degree, still do). In Mahouka, pretty much all girls have their own appeal, and while I have my preferences, I don't have any dislikes.
As a character, Mayumi is great. Intelligent, playful, yet easily embarrassed, and I really like her genuine efforts to help people out and overcome the stupid bloom/weed divide. If she were to end up with Tatsuya, I would only mind that there's a worthier girl who has suffered through much more and tried much harder.
Mari is awesome (and hot as hell) and makes a great pair with Mayumi. What's not to like? But since she is already attached, there's not much shipping involved here
Miyuki is a special case for me. Sure, there are sides of her that I normally tend to disapprove of: Her possessiveness and jealousy. What more than offsets them however is my knowledge about her background and past - what she went (and still goes) through would emotionally warp anyone, yet she managed to stay kind and positive throughout. She's the girl who is going through the biggest ordeals to support and protect Tatsuya. And so, she's pretty much the only imouto character I can think of that I fully root for.