My answer doesn't pertain to
Avis specifically, but strangely enough, I've had the same company (Dollar) treat two separate rentals, one in Las Vegas, one in Orlando, differently, last year (the rentals were only about a month apart). In Vegas, they didn't charge or put any kind of hold on my card at ALL, which I thought was pretty scary for both of us, since I could've used that money and maxed out my card. I'm sure they could just "overdraw" my credit, but that wouldn't have been a good thing for me.
In Orlando, they put a "hold" (they don't actually "charge" the card until you return it, but they can put a hold on a certain amount) only on the estimated amount with all the taxes and stuff. Actually, something got screwed up and they actually put two holds for the same amount, but I'm sure it was supposed to be just one.
On some rental car websites, I've read things to the effect that you should be prepared to have something like $250 above the estimated total available on your card, or $500, whichever is higher. Again, they will place a "hold" on that amount so you can't spend it elsewhere, then they will charge you the actual total upon the return of the car. I think my experience in Vegas where they didn't even put a hold on my card was an error (the whole experience was bad, so it wouldn't surprise me if they made a mistake! But that's another story...).
So far, most of my experiences have been where they only put a hold on the estimated total (although a lot of websites say be prepared to have a couple hundred dollars more on hold).