Is now a good time to plant cool weather crops?

This is my second year growing a food garden, so I'm still pretty new to this. Last year I did mainly transplants, but this year I want to grow some plants from seed that I didn't grow last year. I've always heard you should get "cool weather crops" seeds into the ground in late February, provided the ground is workable, which it is. I was thinking of prepping my garden with compost and fertilizer this weekend, then planting carrots, lettuce, and onion seeds next weekend. I live in northern AZ, so it doesn't get too terribly cold here in the winter (average February temps. Are in the 40s, and average March temps in the 50s). We will still get a few more frosts. Would next weekend be a good time to plant those 3 or should I wait a little longer? (I don't want them to be killed by the scorching heat in the summer by planting too late either).