Can I get better hotel deals in Vegas by just showing up at those hotels and not pre-booking?

I would find and book a hotel room in advance at the rate you find that looks reasonable to you…note the cancellation policy of that hotel. Then, on the day of, I’d walk into a few hotels and see if I could get a better deal. You will be walking the strip anyways, why not just drop in and ask. Another way to get a deal is to ask if the hotel has any rooms on maintenance that they’d be willing to give you at a discount. Maybe there’s a room without a TV in it or something like that. If I did get a better deal then I would cancel my other booking (following their cancellation policy). You’re covered both ways.

Yes, sometimes hotels do have same day deals, and sometimes the discounts can be very deep. Most of the time these are caused by an event or group canceling or the hotel over-estimating the demand for that day. Hotels forecast their occupancy out weeks to months in advance. If there is a day or days where they will significantly miss their projections, you can bet that they’ll want to put you in a room and in a hurry.