Yep, March is busiest time. I pretty much don't leave my house during the spring because there are so many people. Early-mid April is still relatively nice weather, and there are less people. April 25th is the earliest day its ever hit 100F in Phoenix, so... you know... book before then.
Also, for anyone from cold weather climates visiting... Please remember that if you go hiking, etc... bring lots of water, a hat for shade, a cell phone in case of emergencies. Every year visitors die hiking in the mountains around Phoenix because they were ill prepared for the warm, dry weather. Please be careful.