I do it quite often on downhills, but it does depend on the bike, and the design of the fork and wheelbase.

The fork is the major factor, in my experience. I have a Fuji Touring circa 2000, and it had the standard fork on it. I had a lot of trouble keeping the bike on track from new, let alone almost never rode it no-hands unless doing 35km/h downhill. Last year I replaced the fork with a Surly one, and the transformation in handling and feel was remarkable.

My new Thorn touring bike has a similar fork on it, and it's much easier to ride no hands at a lower speed -- around 20km/h -- loaded and unloaded.

My rando and road bikes are less so. And I won't take the risk on the tandem.

Sitting back does help.