The question is asked in late January, so the crape myrtles should be pruned before new growth begins in spring. The plants would do okay even if pruning were to begin after new growth appears, but you would run the risk of damaging tender sprouts.
Pruning crape myrtles is practically a late-winter tradition in the South. But it's not always a good idea. Take a look at the accompanying photo of a 'Natchez' crape myrtle. Apart from, perhaps, a little early training, it has not been pruned. Yet, the form is very pleasing. Also, check out this photo of a group of 'Hopi' crape myrtles. They have never been pruned, but are beautifully shaped, and naturally so.
The style of pruning many homeowners have in mind is called pollarding. It comes from the verb, poll, which means "to cut hair." Pollarding is an ancient practice that began as a method of producing fodder for cattle and harvesting firewood. But pollarding crape myrtles is for the purposes of producing larger flower clusters, and for reducing the size of the trees. Size reduction is usually called for because the crape myrtles were planted in the wrong place to begin with.
Take a look at this pollarded 'Natchez' crape myrtle that was planted too close to a building, along with a close-up of a pollarded limb. It looks like a gnarly fist.
This is another 'Natchez' crape myrtle, also planted to close to a building, that awaits pollarding. Notice how long last year's shoots grew - very unlike the unpruned 'Natchez'. Obviously, once you begin pollarding, there's no turning back. You'll have to do it every year thereafter. It's nearly impossible to return the tree to its original attractive form.
Whether pollarding improves the appearance of the tree is a matter of opinion. But I advise that if you don't need to reduce the size of a misplaced tree, leave it alone.
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