There was much debate about this awhile back, and although our experiences aren't definitive, I thought I would pass it along, anyway...
We had a letter from the father.
Victoria was never asked for it.
Also, she did something at the police station (I think) and had them write Stas' name in her passport. (I'm not sure what that was all about)
However, having said all of that... Victoria feels that anyone SHOULD have a letter, because some customs/passport officials can be very thorough, and she thinks it is very possible that she COULD have been asked for it.