If you are using external processes (tools like gd's converters and so forth) yes. For code that is only running in node's environment it used to be that running a process or more per core (using something like nginx as a reverse proxy to tie them to one port) was considered the way to make better use of extra CPU resource.

There is even a cluster module built in now to remove the need for an extra external tool to manage the processes: http://nodejs.org/docs/latest/api/cluster.html (there are more fuller featured options available as extra modules, I'm not sure how they compare efficiency-wise with the in-built one). I'm guessing this isn't the way to go if the processes need to communicate, but I've not looked into it overly deeply yet (my experiments with node not having grown to the point of needing to take advantage of more than one core).