The nice thing about renting cars in Vegas is that most of the strip resorts offer free self-service parking, so unlike most big-city hotels outside of Vegas that charge you a ridiculous $20+ per day to park a car, you don't have to worry about that in Vegas, both where you're staying and where you're visiting while staying. Hell, if you don't mind tipping a few dollars, most of the resorts offer complimentary valet service as well so you don't even have to find a parking spot; just don't be 'that guy' and not tip. If you're going to be in Vegas for more than a few days, I personally like having the rental, because every time I go to Vegas I either stay on the strip, or off the strip, but want to be in the other place several times, don't like waiting for busses, and also tend to go to Fremont Street at least once. Your taxi from the airport alone, and back when you're leaving, will typically be from $22 to $40 each way depending on which end of the strip you're staying on. Going from a strip resort to Freemont Streeet will be upwards of $35 depending on where you are. Going from Palms/Rio/Orleans/Platinum or other popular off-strip resorts to the strip will often be $10 each way. For more than a few days, you can easily go over the $200/week (all in) it will cost to rent a car.
Two things about the rental car area of McCarren airport:
The rental car bus pickup is right outside the baggage claim area, so it's fairly easy to get to without hauling your luggage a mile or two. They'll take you to a depot where all the rental car companies are located. The depot has free luggage carts so if you've got a bunch of bags it's easy to roll them all in while you go through the line of whichever rental car company you're going to use. I can't remember if the airport itself has the carts for free or if they cost a few dollars, but they are there.