There should typically be no need to cut back Crape Myrtles. Pruning forces new growth now and we are near the end of the growing season, so early frost could zap the new growth. However, when dormant, you could do some minor selective pruning: cut branches that cross, branches that were completely dead, etc. Feel free to cut suckers at any time. Some people also grow them as bushes instead of like trees.
Some varieties of CM will rebloom if given a short trim haircut all around, maybe just a few inches. But do that immediately after they stop opening flower buds. Other varieties may not respond and rebloom.