Our FS told us what he prefered to do and why. We could of, if we wanted, objected but he is the specialist and so went with his advice (which has resulted in 1.5 kids from blastocysts so we're very very happy). So it was set as the protocol but I also understood the embryologist also had input once the eggs were in the lab.

it's your eggs, your body so you definitely get a say... you can definitely question them as the decisions, at the very least you should be informed about why they think it should be day 2/3/5.

hth