There seems to be some controversy on whether the number of jobs in GNU make is supposed to be equal to the number of cores, or if you can optimize the build time by adding one extra job that can be queued up while the others "work".
Is it better to use -j4 or -j5 on a quad core system?
Have you seen (or done) any benchmarking that supports one or the other?
I would say the best thing to do is benchmark it yourself on your particular environment and workload. Seems like there are too many variables (size/number of source files, available memory, disk caching, whether your source directory & system headers are located on different disks, etc.) for a one-size-fits-all answer.
My personal experience (on a 2-core MacBook Pro) is that -j2 is significantly faster than -j1, but beyond that (-j3, -j4 etc.) there's no measurable speedup. So for my environment "jobs == number of cores" seems to be a good answer. (YMMV)