I've had two standard tests for pre-employment in my industry, tech support.
Both times I tested positive for diazepam. I was currently taking hydro and Valium daily. Didnt matter because I had a prescription. The company that performed the test called me asked me if I had a prescription and told me to fax a copy of the receipt or a copy of the bottle to their Dr. (I had to do this because they were not allowed to verify with my Dr or pharmacy).
On the other hand, I went to college with a kid who runs a business that sells drug testing service to business. I was told percocet usually shows up but vicoden doesnt. Also, my wife took a drug test for her job at a large insurance company. Xanax didnt show up and it had been three days since she last took it, but she only takes it occasionally.