welll.......

Flowers and seeds take energy, so if you prevent the plant from spending the energy on the flowers you're likely to get more growth. However, crepe myrtles are pretty danged tough, so I wouldn't really think you need to cut anything off -- they're gonna grow no matter what. I might go ahead and cut the seeds off, though.

Cutting the seed off after they bloom will let crape's bloom again sometimes.
Norm

Along the lines of what Norm said, I think that if you cut off the blooms, it might trigger the shrub to put even more energy into producing more blooms.

But, removing the seeds after the blooms have faded, like Amazindirt said, would probably be alright. Many plants aren't encouraged to reblooms with seed removal like they are with flower removal. This is a general guess, based on similar plants, on my part.

OK... I'll wait for seed. Thanks!