I think as with most characters in the books we don't know enough about Minerva to say that for sure. Take the exception for Harry to play as a first year for example - she overstepped the rules because she saw an advantage over Slytherin, that is blatant favoritism. Interaction with other houses is not displayed in the books and I believe that with reason. The books themselves are coloured with favorable bias towards the Gryffindors :)

But as this universe is set after the war, I have the hope that Minerva has learned from Serverus sacrifice and now treats all of the students equally, no matter what their house.