Should you do it and how much without hurting the plant?

No.

A friend of mine had two rows of crape myrtles lining her driveway. She trimmed only the spent blossoms off in the winter on one side just to see what would happen the next spring. It took her 6 hours to trim the plants.
Next spring, there was no difference between the two side in terms of blossom quality or quantity.
Trimming is not only harmful and time consuming but it has no visual effect.
If you want to trim, cut the sucker shoots that come up from the root that hide the beautiful trunks.
Tony M

Ultimately, it is whatever you want to do and how much work you want to put into it. The only thing I would not recommend is "topping" them, as it results in a scarecrow look in winter and it's hard to shape into a normal looking tree after that is done.

Here's a spirited exchange from last year, including links to pruning techniques and some pics. http://dirtdoctor.com/forum/viewtopic.php?t=2642
And, after the first freeze is just fine.

Good luck!

Ok...don't jump too fast at that subject line. I moved into a new home in Rockwall and the builder did it...not me. But.. I do have two crepe myrtles that are too close to my home so I have to cut the limbs that get against the house and/or eve of my home.

Now that that's over...I have never cut crepe myrtles back except for the mentioned "have to's" But this year they didn't bloom as much as in the past summers. Is this because of the wet, cool summer we had?

YUP!