Personally, I would not do Vegas without a car. Yes, you can take a taxi everywhere. BUT traffic is terrible on the strip and if you get stuck in traffic, a one way fare costs you as much as a day's rental. Also, if you stay someplace like the Venetian you are on the opposite end of the strip from the airport, so count on spending at least $20 each way for a taxi. Plus, it is HOT in Vegas in June and the blocks are REALLY long, so getting from one hotel to another can take a long time. Bring comfy shoes and water if you choose to walk. I would watch for last minute deals on rentals. Last fall I got a minivan from National for $45 for Fri-Mon using a last minute deal and a $20 off/weekend coupon.
If you decide to rent a car, definitely look at some of the off-strip hotels. You can get a much nicer room for way less, just by staying a block or two away.