Safety is a tough issue in today's society because pretty much everyone is looking to win the "law suit lottery" if they ever get the opportunity. Very few businesses will assume the risk of officially saying that something is safe. Everything gets warning labels "for off road use only" etc, etc.
Lots of people will give advice on the internet, since they assume they won't be held responsible for any injuries or loss of life. In my opinion, it's your responsibility to make safe decisions. When you modify your car, remember that a mistake could kill a nearby child on a bike or a family in their car.
Ok, sorry I'm going on too much here. I'll cut to the chase.
Wheels do not hang from the studs. There is a circular protrusion called a hub that centers and supports the wheel on the spindle. If the spacer is thin enough that the hub still supports the wheel, in my opinion that is safe. It's still a good idea to use new, good quality wheel studs with the spacer.
If the spacer is too thick for the wheel to be properly supported by the hub, the spacer must have a hub built into it. In my opinion a thicker spacer with a hub built in is also safe, as long as the spacer is well made. I recommend inspecting a thick wheel spacer regularly. The regularity depends on use. If you are racing the car, don't use a spacer! If you like to corner hard and you drive on rough roads, I would think you should pull a wheel at every oil change and look for cracks in the spacer or bent studs. I wouldn't pull every wheel, just a different one each oil change.
Keep in mind all this advice just came out of my head. If you buy some junky cast spacer with all kinds of porosity in it, it may fail when you go to bolt it up. Please think and be careful and take responsibility for your own safety!
