As the weather here is Great in the winter and it's Cold most everywhere else, yes Jan thru May rates tend to be high.
Plus there are so many major events here starting with the Barrett-Jackson Classic Car Auction and a few other classic car auctions at the same time. Then the ball players are heading in for Spring Training baseball...then the Games Begin in Feb, major Prime Month. The Arabian Horse show lasts several weeks. People from all over the world come for that.
Winter is SEASON here so I wouldn't expect your rates to go down. With the economy the way it is, it's hard to tell if all the car rental companies will have as large a fleet as normal out here so that might have an affect on rates as well. You know, sell out faster and then the rates will be higher on available fleet. You can liken that to hotel rates or airline rates being higher when there is less supply available. It happens with car rates too.
So the best advice I can give to anyone for PHX...book early to make sure you even get a car as we are moving into the busier months now. We've been selling out quite frequently. But we have a greater variety of makes and models than we've ever had before. Seems like the only maker we don't carry is Honda. I've seen almost everything else cross our path of late.
Make sure you get those Hotel Reservations booked Too!