This may only apply to cartridge bearing headsets but Cane Creek publishes a factory preload recommendations of 10-15 inch-pounds and Chris King recommends 4-10 inch-pounds measured at the top cap tension bolt. This isn't a lot of preload but it isn't "none" either.

Tightened to these specs, the spacers under my stem are very difficult to rotate and the fork has no play when rocked against a locked front brake but the fork turns freely with no drag or binding. The CK headset I've always tighten to their recommended preload has 37,000 miles so far and shows no signs of wear or roughness.