I think you definitely need one, because they can get lost in the shuffle. My sister forgot to bring the card box she made us and all night people kept coming up to us to give us cards.
It is a good idea to have something for people to place their cards in so that they aren't all over the place (especially since they can go missing very easily).
There are many type of card boxes you can do. From what I hear, most people prefer something that is closed so that people can't easily reach in and take anything. Hopefully no one would steal from you on your wedding day, but better to be safe than sorry.
I took a wooden crate (bought at Michael's) and made a cute "cards" sign for it. I also put ribbon around it in our wedding colors. It was open, though. I don't think anyone stole any of my cards.
There are a variety of ways to keep them safe and organized. Even a beautiful cloth bag that goes with the decore...but I would "assign" (of course ask nicely) a person that you trust to kind of keep a handle on it. Not that anyone would steal them...but to ensure that none get tossed in the trash by accident. At some of the "higher dollar" weddings that I have been to a groomsman or aunt or whatever would kind of "man" the gift/ card table to make sure that cards stayed where they were and were accounted for...seems like a good idea and a way to include someone that may have not had a big roll in the wedding. We are doing a vintage picnic basket...and I am sure my dad will keep an eye on it. LOL
This idea is new to me - does it not come across that you are expecting something ? If not would I need one for a small wedding or are they more for when you've big
There are some really adorable boxes on Etsy, though as with all things on there, you've been warned that it's a rabbit hole. FH and I are now debating whether to spend $200 on a little "doghouse" box with a hand-painted image of our pup (she "helped" him propose to me, and we're thinking we could use it for her treats afterwards....someone please help me justify this?!).
Yes, you do need a card box. People will bring cards and there needs to be a place to put them. It's also a good idea to think about safety - unfortunately, not everyone is honest and there are lots of stories out there about cards being stolen out of unsecured card boxes.
DH and I made ours. This is before it was complete, but it's the only picture I have right now. I lined it with fabric and DH made a solid lid with a slot in it that was nailed on top. And... I totally agree with twostep about Etsy; a similar "beer crate" box goes for $150 there and we put less than $15 into this one.