Answer: Crape Myrtles bloom on new wood so to encourage lots of flowers, you'll want to encourage the growth of lots of new wood. The best time to prune these trees is in the spring, right after they begin to leaf out. There are several ways to prune Crape Myrtles, depending upon the look you want:
Many people including some professional landscapers just arbitrarily pick a height(for example, 6') that they want their tree to be and every spring they get up there with a hedge trimmer or chain saw or whatever and "whack off" everything above 6'.
The plant then makes this flush of new growth and covers itself with flowers. Many people prefer this way and think its the way a crape myrtle should be pruned. If you have a group of plants in a line along a driveway this method makes the plants very uniform in height.
Pruning in this mannner would ensure that your trees never got so tall that they would block your neighbor's view.
If you decide to cut the trees down to ground level, you very well may end up with dozens of upright shoots growing where a single trunk once was. This rampant vegetative growth will be long and supple; branches that develop eventually will bloom, but their wood will be so limber that the big flower heads they develop will droop downward.
So to answer both your questions, yes, you can trim your trees in just about any fashion you please. Now that you know the probable results, you can decide just how best to trim your Crape Myrtles.
Q&A Library Searching Tips