Justanothermile, in terms of being around, that is a decision for you and your DD:
For your part (obviously the candidate too, but you get my point) there is lots of waiting around. So if you have time to kill, and don't need to be anywhere, then you can mooch round the city, or drink a lot of coffee somewhere.
Then on her side, is she the sort of person who would appreciate downloading with you over a spot of dinner before heading back into college for a sleep? It would not have helped me to have either of my parents around, but I'm sure we'd have been sitting in sullen/depressed silence, picking at food, before I moped off, if they had been.
Equally if it's a long journey home, your DD might appreciate the company, or might wsih to be alone with her thoughts, to rehash the perfect answer they should have given, for the 23rd time.
I'm sure you already have, but take a look at the December posts at last year's threads, and see if you can spot your DD among the candidates, and what the parents were up to. Would it have been useful for you to be around post potential meltdown, to pick your child up? Or would it have just further distressed her?
I would lean to the side that most on this thread have 'I'm only a phone call away, and have a safe train journey back!' but for some candidates they'd probably be delighted to see their parents after such a nerve-wracking experience.