It all depends on the kind of wedding you're having. I went to a wedding with 30 people, so there really wasn't a need for assign seats. It was also really informal. But, when you have 100+ people you really do need it to avoid confusion. It also helps you organize favors, to be sure people show up since you may be paying per person, etc.
For us, we are assigning seats. Personally, I think it looks nicer and I like to be organized. It also helps so people aren't moving chairs around, etc. We also need to assign seats because we have to have our guests choose their meal ahead of time and are letting the wait staff know.
It isn't bad to not assign them. You will save money on table numbers, escort cards, etc. The choice is really up to you and what you want to go for.