If you don't mind walking, there will be plenty to do and see withing walking distance.
If you want to hit many
casinos all over town (or make a trip to Hoover Dam for instance) you might want to consider a cheap rental car. This would especially be prudent if you can get a car cheaply in a package deal with your air fare/hotel and it will be available upon your arrival at the airport.
A cab trip to and from the airport will total $40 or more, plus tip.
Cabs will be $10-$20 each way (plus tips) for trips of any distance. This can add up quickly if you're taking cabs multiple time a day.
Every casino has valet parking (usually free but tipping is expected) but the traffic can be unbearable, especially if you aren't familiar with the area.
I guess it comes down to personal preference. Let the cabbie drive and pay a bit more or do the driving yourself and save a bit.
Unless you want a car for sightseeing or skipping from casino to casino, my recommendation would be to wear comfortable shoes (no flip-flops) and hoof it.