This is the best advice on pruning crepe myrtles.
"Crapemyrtles bloom on the current season’s growth, so they can be pruned in the spring and will still flower normally throughout the summer. Larger, heavier flower clusters and rank growth often result when the branches are cut back severely, and these flowers are often so heavy that they bend toward the ground. For this reason, we do not recommend the practice of pollarding or cutting the branches back to stubs every year."
We had two crepe myrtles in our back yard in CA. Every winter I would trim the branches back about 12-18", but never severely back to a junction. Here is a picture of the two of them.
Attachment 2095