Jenny maybe I misunderstand the point you're making? We are "not" saying Female Boxers are aggressive by nature ( though my understanding is that are the Dominant sex in Boxers?)I have not had aggression issues in female boxers (only have had 2, so not best gage) but what I have noticed, more what I would liken to ... bratty, jealous, stubborn, 'its not fair, i want THAT toy' kind of behavior.
Maybe when she tries to get toys from the boys, you could try 'no, leave it' and if she doesn't then she gets a time out? Then like others said, you won't lose the effectiveness of the drop it command?
Any cautions about Females was in regard to two Female Boxer households! Just not a good idea! Wanted that to be clear. I loved my baby girl and my next Boxer will be a girl, the one and only Female in my multi dog household.
Bad boys I can deal with Bad Females??? I'm out!