Do you live in a climate where the ground freezes and thaws? Or where there is softer or decomposing materials in the soil? If so, a good compacted base is really important to avoid the patio from warping and settling. Or you don't have to do it, but plan on re-leveling it quite often... It all comes down to what you expect of your new patio: more work now saves work later.....

Even in an environment without a freeze-thaw cycle you should have some amount of gravel for stability. Here in the Chicago area we would be compacting a 7" base of CA-6 gravel in most circumstances.