Genuine question - why assign seats at all? In my country (weddings with 200+ people are the norm) people sit wherever they want. In the US, you wouldn't assign seating at a house party (right?), so why do that for a wedding, since a wedding is after all just a more expensive party?
The trick is to combine guidance with a relaxed atmosphere where it's fine to ignore it. Your dress code might say "black tie", but it's great if someone turns up in jeans because that's what makes them comfortable.