I would do it now, before it gets hot or wait until fall.

Dividing them now could have an effect on how many blooms you get and how big they are. If that isn't an issue for you then now would be fine.

Thanks aesarien and hollyclyff for your responses-- haven't gotten to dividing them yet... planted 200 begonias today and got the vegetables in Monday--- Thinking about dividing them tomorrow-- if my back holds out!!!

If you divide them now, cut the leaves back to 5-6". Don't expect blooms this year. I would let them bloom and divide them in the fall.

Thanks for the responses, been out of town for a few days-- man is it hot here--- not even going to try to divide until fall.

thanks though