Step off during stretch - Balk or not?

[mach3]

We had a discussion about this one, and I would like to get your opinion.
If a pitcher does the stretch, can he step off during the stretch or not. So if he is bend forward to look into the signs of the catcher and he starts to go back to bring his hand together. Can he step off befor bringing his hands together? When is he allowed to step off?

Thanks

Raoul