IMO, start checkig now and book when you find a decent rate. You can even book the lowest you find now and then keep checking... As you get closer, you'll see the prices go up and down. Grab a new ressie when the price is down and cancel your original. I'm going April 26th. So far, my best rate (with Alamo) was reserved in November. Right now, the rates are quite a bit more. I'll keep watching and as we get closer, I hope to find some great deals.
At any rate, it doesn't hurt to create a reservation as soon as you find something you consider OK -- just keep watching for something great to come along!
HTH!