Having a receiving line at a wedding is a tradition in many cultures and families and it is a great way to make sure that the bride and groom meet and greet all of their guests, just in case they don't have time to make rounds of all the tables.
However, there is no such thing as the average family and therefore there is also no such thing as the average receiving line either. Some brides and grooms will have parents who are divorced, remarried or unfortunately deceased and this can dramatically change the dynamics of a wedding receiving line.
If your parents are divorced or remarried or both, the receiving line is probably just one more thing you have to alter to make sure no one is offended, left out or seated next to someone they don't like. Whether you have your new step parents in the receiving line depends on your relationship to them, as well as whether they are hosting the wedding - that is, whether they are paying for some or all of the event.
With regards to placement, you can put an attendant - a bridesmaid or a groomsman - in between your divorced parents or in-laws, or you could put a divorced mother of the bride next to the groom's parents or vice versa.
The role of the bride and groom in the receiving line is to introduce guests to their parents, their in-laws or each other, whom they may not have met, or don't remember. While all you need to do is say a quick hello, thank your guests and give them a hug and a kiss, receiving lines are estimated to take around 30 minutes per 100 guests.
Therefore, you may choose to greet your guests as they are still seated in their pews after the ceremony and this works quite well for weddings with around 150 guests as no one has to stand for long periods of time waiting to greet the bridal party and you have the reception free to talk to the people you actually want to talk to. Other brides and grooms use their pre-dinner drinks as an opportunity to introduce themselves and their new spouse.