Define Router. A router is only a firewall if it has access control on it. If you have something like a dlink or Linksys 54g router and wireless access point, you have a firewall for the wired portion to the internet, but not wireless. Your wireless clients can still be vulnerable as can your wired clients through the access point.
Is it a big risk? No. Well, unless you live in Vegas near the hotels during Black Hat

But be careful about Tootsie Pop kind of security (hard on the outside, soft in the middle) as it just takes one bad apple to mess up the inside. You are reasonalby safe at home (by obscurity), less so at a school.
Me, I run the firewall at home on the clients. Layers. But I am paid to be a little paranoid.