The GA Extension Service leaflet has lots of information on the subject.
It also includes a very useful chart of the different cultivars, colors, sizes, disease resistance etc.

Here is a link that might be useful: Crape Myrtle Culture UGA CAES

You do not "need" to cut anything off. New flowers would come regardless of whether you leave old ones.

The pruning rule I think the best (after not pruning!) is don't cut anything thicker than a pencil ... unless you are pruning to correct a shape defect.

Each winter I look at the bare tree and just remove limbs that are crossing or rubbing, stuff like that. I remove them completely not lob them off. If you can easily reach, you can remove the seedheads just when they finish blooming to increase the blooms, just like deadheading an annual flower, but it is not necessary. Most trees are too big to do all that, so just leave them there as winter interest, and leave the tree to it's natural shape. I guess you know that severe pruning is called crape murder.

Don't prune unless you have some rubbing branches or branches that are wanting to grow inward. I just leave the old seedheads on.

I can't stand to see them chopped off, don't know why people think that is a normal thing to do.