In my experience - in having done one CP tour myself and in having talked to people who've done multiple:
When you go down the first time, you're pretty much at the mercy of the recruiters and management. You can request a specific location, and sometimes they ARE able to accommodate you, but, hypothetically speaking, if they want you at Mission SPACE, then you're working there no matter how badly you wanted Test Track.
Once you've completed one tour, you have a *bit* more negotiating power, in that if you've worked a location in the past, and if your management team really liked you and puts in a good word with recruiting, then if they *can* they're often willing to put you back in a previous role. They can't always do it, but they'll often at least try.