As a general rule, never pay a prossie in advance would be my advice. It is usually unnecessary (even those who ask for one can usually be persuaded to forego it) and is highly likely to be a scam.
However, the world isn't black and white and I would consider exceptions.
As an example, I once had correspondence with a prossie from Norfolk, I think, when I lived in Exeter. I wanted her to come to me, as I was a lazy arse, but she, and not unreasonably, wanted a deposit. It didn't happen in the end, but not because of the deposit issue.