Judging by the post above, the $20 trick apparently can work at Wynncore. You really have nothing to lose by trying.
My technique is as follows: Establish rapport with the front desk guy/gal, I usually aim for a younger looking guy (I'm a guy too). As he/she is working the computer, ask if there happens to be any better rooms available (state the special occasion if there is one). As you ask this and continue your chit/chat, open your wallet, and discreetly hold a $100 bill in your hand (ensuring he/she sees you pull the bill out of your wallet) but do not hand it over. I'll usually roll it up in my hand so that the managers or other employees won't see it. If they say there isn't anything available, then you lose nothing, and there is no awkwardness in having the money returned to you. If he/she gives the upgrade for free, you're a winner and you hand the money after the transaction and you receive your room keys as a "tip".
It's not a bribe, ok well maybe it is. People may call me cheap, but I actually think this is a really fun way to start a trip in Vegas by "gambling" a bit at check-in. It's not that I don't have the money to pay for an upgrade upfront, it's that if I can save $200+ a night on a room, why not give it a try. If they decline any complimentary upgrades, you can always pay outright at the front desk. I don't see any value in booking a higher level room several days in advance.