Correct...in many cases. Here's a reference to consider:
http://onlinelibrary.wiley.com/doi/1...50507/abstract
Regardless of "how dangerous", if the risk is only 1% are you really willing to gamble with your life? Aspirating takes a second. Why risk your life for a quick second?
Regarding doctors and nurses, they are far from perfect but many of them have far more experience injecting substances than most of us. Again, they're not perfect but having injected and drawn blood from both animals AND people, I can tell you that with experience you get a "feel" for where that needle is even though you can't see it.
IMO its well worth the time to aspirate.