leaf curl on crepe myrtle
Should I spray and, if so, what should I use?
You'll need to pin down what's causing it. Have you checked for aphids? It might also be suffering from Verticillium Wilt.
These articles may help you:
Inspect the tree to determine if there are any insects that are causing the leaves to curl on your tree.
Neem Oil can be used to treat most sucking insects.
Here are several articles that will help you pinpoint the issue.