I spoke to HR about that. They said when medical gives the O.K. and clearance/etc, Is good, they'll send an final offer with the date and facility type. I told her I wanted center, since that's where all my experience is at. She said they need more in center, so that would be easier to do, and that she would put me in center. She warned me that doing so could possible mean giving me a later start date than terminal or vice versa... Not sure if she actually will, or if she was just trying to get me off the line.
So if you want center, that'll probably be easier to do. But if you want tower, and they don't give it to you, of course I would ask them anyway. Just depends on your rep. If you're CTI, and tell them your school emphasized in terminal (even if it's bullshit), that could help. Also if you have ghost pilot experience in terminal, that may sway them. I've heard of people using both of those excuses.
Some bids they're flexible and a accommodating, and some bids they won't even consider it.