Another tip I have gotten on this through working in events, is if you use tent place cards, write the name of both side of the card, so that way if you do have people on tables who don't know each other, they can have a sneaky look to learn the names!
FH has some family that we want to keep separate - for reasons such as not getting along, or people who drink too much when together. Also, it would mean a lot to my ILs if they had a 'special table' with 'honoured people' on it. Whereas my family will all fit on one table anyway, haha.
I alo want to keep my work friends away from Fh's cousins... so.. yeah, I'm having set seating because I'm a control freak.