That's probably good enough for most use cases, this is really another case of different cases have different needs and security isn't magic. So in the spirit of do what you need think about what a firewall is, what the drawbacks of running one are and what risks it mitigates:
- A firewall is something that restricts the flow of network traffic
You mentioned your router has a built in firewall, this handles 95% of what most linux users would want a firewall for. I assume whether you realize it or not you probably also have some iptables rules set up as a software firewall.
- What's the drawback?
All of network/computer security is a balance of security and convenience. Firewalls generally aren't that inconvenient until you get into add source restrictions (the static IP address of my office can connect but everything else is denied access). So you probably have next to no drawbacks.
- What risks does it mitigate
More assumptions here: You don't run your linux box on the public Internet, it's in a private subnet (192.168.X.X probably) and behind a NAT'd modem/router. This means you're probably mostly secure from the bulk of threats.
All this to say, in this day and age most home/workstations users shouldn't care too much about maintaining their software firewalls on the machines they use. That doesn't mean firewalls aren't important (they *really* are), it means your router is handling that for you already for the most part. Systems administrators and production servers are another story, if you're a professional sysadmin you should know your firewall and actively maintain. The open Internet is a scary place and if you don't believe that look at your auth logs ;).