Honestly, it is warm enough to go all year round. The best months to go is late spring, early summer which is in December. I take that back actually. If you are talking about São Paulo based beaches and beaches in the South, I wouldn't go during the winter because it is a bit cool.
My wife is from São Paulo and she always tells me how the beaches in Ubatuba. are nice to go to. There are beaches just outside of São Paulo City that are supposed to be nice from October-April. The Santos beach is one she doesn't recommend at all. I guess if you want to go to SP beaches they are all in the interior. We will be going to São Paulo in July to renew vows in front of her family.