Just moved to Houston from Santa Fe, where I had a pretty nice garden. I'd say fall is the best time to plant trees. I had mahonia, maple, desert willow, western catalpa, chokecherries, lilacs, several apple and plums, and some pinion.
The most important thing in Zone 5 is to plant trees that will thrive in the alkaline, caliche soil that exists there.
The trees also need to set up a good root system before they bud out. If you plant in fall, the trees will go into dormancy (at least what's above ground will). The root systems will continue to grow.
Spring is not a bad time to plant, but the warm weather will encourage the trees to put their energy into sending out shoots and greenery, instead of establishing a root system, which is of utmost importance in such an arid area.
Although many nurseries encourage people to amend the soil in the planting hole, I (and many others) argue against this, and opt instead to plant native trees whose systems are acclimated to the dryness, the alkaline soil and whose roots have learned to penetrate the caliche hardpan, and seek out precious water.
There are also other ways which you can encourage healthy root growth.
One way is to direct rain runoff from your roof into things like dry stream beds which then directs the water to your trees.
There are several other alternatives to this. There are some great publications available. You may also want to go to http://www.highcountrygardens.com/ezine.html (that;s in Santa Fe) for information about native plants.

Here is a link that might be useful: Desert Gardens