I landed in Vegas at 1 pm Sunday 2/1. Had a car reserved at Dollar thru CarRentals.com at $20.61 per day ($91.49 with tax & fees). When I was checking online, Dollar's rate was cheaper than most of the others by about $2 per day. When we got to the rental car center, there were at least 75 people in line at Dollar. The line was barely moving. I checked the walk up rate at the nearby Budget counter but it was $39.99 per day plus tax & fees. Luckily I had their 800 # with me and was able to call for a cheaper reservation. I think i got $29.99 per day at Budget which was $119 with tax & fees. It cost me $30 more, but I'm sure I would have waited in that Dollar line for 2 or 3 hours so it was worth it.
So my advice is to carry the #'s of several of the major rental car companies with you , and to make reservations (neither company asked for a credit card to hold the reservation) with more than one company so you have a backup plan.
I know it's pretty basic advice, but think it's worth mentioning.