Seating protocol for a wedding rehearsal dinner is at the discretion of the bride and groom. The couple have the responsibility to choose whether they want to assign seats at the dinner or allow the guests to choose their own places.
One bonus of assigning the seating at a rehearsal wedding is that it encourages people to connect with those they do not already know. Ice-breaking activities are also useful at a rehearsal dinner because they help give newly introduced guests something to talk about and a way to get to know one another. The bride and groom should feel free to choose whatever option they think is most comfortable for their guests; a smaller, more intimate wedding may not require assigned seating, but a larger affair might necessitate more organization.