The scan only represents a snap shot from the time the test was ran, with that in mind who is to say that one of those ports may become exposed with the launch of a application or service on your system.
A big part of firewalling is outbound protection which you currently have none of, you may become infected by a virus/trojan or any type of malware that has the ability to provide a back door to yor system, with a firewall in place you stand a chance of preventing that exposure.
Most test sites only scan common ports, you may be exposing something above the 1024 mark.
I would overall say if your on a high speed connection then you should firewall the box.
However, if you keep it patched and secure and use the correct Restrict Anonymous settings then you system is OK.
I recommend you go for the higher level of security unless you plan to keep a close eye on internet activity and activity of apps on your system.