If I'd be desperate for a specific gender I would do it.
We have 3 boys between us, tomorrow is a gender scan for this baby, and, although I would like to have a girl now, if it is a boy I'd be equally happy.
But, if it is a boy - we are trying one more time (hmm... probably if it is a girl, we are trying one more time anyways, I want 4 kids

)
What I DO want to say, I DO not view choosing a girl embryo over a boy embryo as a killing of others... If couple goes through IVF, they usually have a few good quality embryos to be transferred back. Which are chosen randomly. So, if I wanted a specific gender, and knew there are 2 male and 2 female, all same quality, why not transfer the gender I want? Freeze the others. And if I decide not to use them later, other embryos would have to be destroyed (isn't it what is being done anyways?).
I could not donate my embryos though... Since they would be genetically mine. But it is personal.
So, on the whole - if you have 2-3 boys and want a girl - I would go for it!
Also other way around.