If you are planning to see more than the Strip in Vegas, a car is a must. Fortunately, the parking in Vegas is pretty easy. They have a free Valet (just need to tip) and self parking. The only place I had to pay for a parking was Fremont (but some places give you a money back with validation).
McCarran offers shuttle buses to the rent-a-car center every 5 minutes. The bus drivers are helpful with your bags too. The ride is about 7 min. When I returned the car, I was so happy because I saw a huge traffic going to the airport but none to the rent-a-car center....probably saved me a lot of time. It took me about 30 minutes to pick up a car (I had a problem with the first car assigned so I went back to change, if not it would have been 10 minutes) and less than 5 minutes to return the car.