In OBR he may turn in the preliminary stretch up to and including while he is coming set. Once he is set he may have a slight and slow turn but it must be slight. As was stated the body isn't a statue so a little is ok. A sharp turn from any position is going to be a balk.
In FED they are less forgiving. As soon as he steps on the rubber he is governed by the rules. I say follow as above with the slight and slow. Anything exaggerated or qiuck should be balked.