I live in Las Vegas, go downtown quite a lot and always park at the Plaza Hotel Casino on Main St.
The reason is that you can park there free for 24 hours, just get your parking ticket validated at the main casier's cage in the casino, you can do that yourself very easily. The other hotel casinos downtown give you only 2 or 3 hours free and you pay per half hour after that. Also you cannot drive off the parking garage because there are high walls surrounding the sides which makes that impossible. HAVE FUN STAY DOWNTOWN AND WATCH THE GREAT OVERHEAD LIGHT SHOW STARTING AT SUNSET!!