You could probably get away without one if doing an informal, buffet style meal. Wouldn't dream of it for a formal sit down meal haha I do think it'll work better with long trestle tables rather than round though. Otherwise it could be a bit of a free for all with people squishing more people onto tables than there should be and potentially some tables with only a couple of people on. Depends how well you think your guests will mingle with others. Personally, my friends all live quite far away so it's not often we're all in the same room at once so I know they'd naturally gravitate to each other anyway so I'm sitting them together on my table plan and you may well end up with the same - people just sat in their little 'groups' meaning anyone not in a group could feel left out.
The problem with no seating plan is that if people just sit wherever they fancy, inevitably there will be spare odd seats and then you need more chairs/table settings than you actually need. I'd assign tables at least, you don't need to do specific seats.