There is no hard and fast rule about when to preemerge in the Dallas area. Way too many variables - weather, what weeds bother you the most, and what product you use.
The end of Feb and the first part of March is the general timing. The preemergents mentioned here -really all that are available - all have limitations and things they do better than others.
For instance, Barricade products take about 30 days to form a barrier to prevent weeds, but it lasts longer. Dimension is my choice in the spring because it has both pre and limited post emergent activity on crabgrass. I also feel like Dimension gives better control on Aster.
The only weed you are really going to control with spring preemergents in Dallas is crabgrass. Most other weeds you see in the spring are late germinating winter annuals - dandelion, poa annua, rescue grass, etc that a spring preemergents won't stop.
In my opinion, applying any Nitrogen before the end of April is a waste of money. Soil temps are just too low - most years- for Bermuda to be growing. No need to fert until your yard is completely green and you are having to mow.
A pretty simple , effective, and readily available preemergent program for Dallas would be to go to HD and buy Lesco 0-0-7 w/PreM and apply it March 1, May 1, Sept 1, and Jan 1. Substitue a Dimension product Mar 1 and a Barricade product on Sept 1 if you can find them. You will control 90% of all your weeds this way.
Fertilize around this program because it doesn't give you any Nitrogen.
