Yep, Rio is a great place for that and he bartenders are cool about it. Quite a few also hang out at the Hard Rock circular bar on the main floor. A few things to be aware of though:
1) Most, if not all, casino bar girls are pimped.
2) Don't ever take a sip of her drink if offered, especially if you're headed back to your room for some fun. They've been known to drug their dates. Not always, but sometimes.
3) Don't ever let a girl bring back a girlfriend with you two back to the room, even if the friend is just going to chill in the bathroom. This smells of ROB activity.
4) Negotiate clearly what you want, and a price, before going back to your room. These girls have been known to upsell like crazy.
5) Try not to pay 100% upfront so that there's some gaurantee that she's going to complete the act.
6) Don't ever let the girl know how much cash you have on you; they'll try to get as much of it as they can.
7) Most hotels have security guards by the hotel elevator. So, be sure to walk up together and act like a couple as much as possible. Some guards will ask to see a room key before letting you up. It's just a formality.
8) Be sure to have plenty of condoms and lube in your room so the girl can't back out of it once you've closed the deal.
8) Never let your guard down around them, especially once your back in the room, no matter how comfortable they may act. Don't do something stupid like go into the bathroom and leave your wallet and/or other valuables out in the bedroom; put anything of value in the safe in your room. Otherwise, they'll be a good chance that the girl, and your wallet/watch/jewelry, will be gone when you get back.
These women will rob you when given the chance. They work for the pimp, not for you. Also, due to the nature of Vegas, they could care less about reviews and/or getting repeat business.
Above all else, trust your spidey senses. If something doesn't feel right, then it ain't.
Other than all that, have fun and write a trip report when you get back.