The last wake I went to, the line went up to the casket, where you could kneel to say a prayer, and then the deceased's 13 children were in line, from youngest to oldest. So, a definite receiving line. And they were all pretty much there the whole time, except for bathroom breaks. They had been hoping to take turns, and possibly run out for food, but the crowds of people coming simply prevented that.
Actually, that wake is a good example for this thread. Most of the people coming knew some of the kids, but not all of the kids. So they'd just shake hands and exchange a few words with most of the family, but maybe stop for a few minutes with the person they knew the best. And the two oldest brothers are priests and therefore pretty well known in the community and a lot of their parishioners, who had never met their father, came to pay respects, so there was a constant tie-up at the end of the line. So the line stopped occasionally, and sometimes flowed around one member of the family who was deep in conversation with a close friend. One person "line-jumping" to join a group further ahead in line really would not have made a difference in how long it took any one person to get through the line.