When? If you do choose a receiving line having it immediately after the ceremony is the ideal option. Waiting until the reception means that some if not all of the guests have arrived before the couple and you'll have to wrangle people away from the bar and their pigs in a blanket and get them to fall in line.
Who? Traditionally whoever is hosting the wedding is first in line. This is traditionally the bride's family but even if all families are hosting the bride's family should come first. So the bride's mother, the bride's father, the groom's mother then the groom's father and then the bride and groom. This is tradition but I prefer my slightly altered way of having the bride's parents and then the bride and groom followed by the groom's parents.
When there are divorced parents and remarriages it's important to communicate before hand what your plans are so no one is surprised to be lined up next to an ex-spouse they haven't laid eyes on since your college graduation. Depending on the general temperature of the relationships parents should stand in line but not necessarily next to each other, for instance Bride's mother and step-father then groom's mother and step-father then the couple and following would be bride's father and stepmother and grooms father and stepmother. There are dozens of combinations, last year we did a wedding where there was a mother and stepfather and father and his husband and the groom's parents all in a row...beautiful!
