As a confirmed and admitted seat cheater.... I fly a lot with my GF, If I am not on BS I still usually have a low A, I check her it at exactly T-24 (that should give her a mid- B at worst)
I take HER carryon with me, I go a couple rows behind the exit row, sit on the window (I like to sleep on planes)
I save her the middle and have rarely been asked if it was open. Also if I take her carry on with me we don't have to worry about overhead space when she gets there.
In my opinion the only people who care about you saving a middle seat, non exit row, halfway back in the plane... are just looking for an argument.