I always rent a car, but I usually try to stay off/near strip just to avoid the traffic. Sure if you plan to stay on the strip, you MAY not need the car. There are a ton of things to do on the strip where you dont need to leave.

If you plan to be drinking the entire time, then yeah a taxi is better then having a car.

reasons I get a car
1)Ton of food places that I frequent off the strip.
2)Some shopping that I can't do here at home.
3)Hit Costco/Sams Club/supermarket for supplies.

In terms of costs. i'm assuming you're flying in, so at a minimum, shuttle/taxi to/from airport. I avoid the Valet all together. You save the $5/tip. But honestly have you seen those lines? I find the walk to the front of most Casino just as long as the walk to the self-park garage. Then your'e stuck driving on LAS VEGAS blvd -which you should avoid. If you rent a car, avoid LAS VEGAS blvd like the plague. Staying at Paris, I think you'd just stay on Flamingo st, know back streets like Audrie (sp?), and Koval and you'd be fine.