I'm seeing an increased amount of vehicles driving around Sydney & Newcastle with empty vertical racks on, so not sure if this is policed (or not).
I've had my vertical rack (early version Stackrack) for 3.5 years. Despite peoples personal opinions that either the overhang is too great, the number plate is obscured (I zip tie the auxiliary bike rack plate on), it has no lights etc. I have never had any unwanted (negative) attention from the police - loaded or unloaded, day or night. Good test was last summer holidays I parked in the middle of several HWP cars & bikes at a rest stop - when the boys in blue finished their lunch I saw them looking at the back of the car, one said to me is that yours? I said yeah...he replies nice bike!
My view is if it looks dodgy it probably is.