I'm not an expert, but I moved into a house with a crepe myrtle 2 years ago.
The first spring the crepe myrtle hardly bloomed at all, and when it did it
was very late in the season and had powdery white stuff all over it. I
consulted a gardening manual that said to wait until Februry to prune it.
Following that advice, I didn't touch it until the appointed time. I waited
until the end of Feb and did some serious pruning off the top (3 to 4 ft)
and cleaned out the bottom a little bit. This year, the crepe myrtle did
great. It was much fuller, bloomed right on schedule with lots of flowers,
and had no white powder.

I can't say for sure that you shouldn't prune now ..but I waited til
February on mine, and the results were so good I'm going to do the same this
year ..

Good luck!
(Dallas, TX)