Hotel probably won't notice or care. Most room rates are rated for two people anyway.
I don't want to burst your bubble... but $640 ($320 for each person right?) for 8 nights still seems fairly low, assuming you're staying on strip. You said you looked at rates on Expedia. Did you factor in resort fees? I can only think of 3 or 4 casinos that do not charge resort fees, and those casino hotels are downtown. If you plan on staying at a major strip property, get ready to spend $30+ per night on resort fees. The fees should be be listed on Expedia's web site, but it's usually semi-hidden in the fine print. So let's say Exalibur charges $80/night for 8 nights, that would be $640. But if you include the resort fee, then your rate would actually $110/night. The hotel will collect the resort fees from you at the end of your stay. Just and FYI in case you over-looked that detail.
Assuming you found a fantastic deal and you already factored in the resort fees, then feel free to completely disregard this entire post.