After my own trial, the hairdresser suggested bringing the BM's to see her just so she could see what length their hair was etc. She played around with different styles and pinned the hair up so I could get the effect but there was no charge for this as it was done on a weekday and in between appointments when they were quiet.
I was delighted with this but TBH I wasn't happy with one of the BM's hair on the wedding day itself so I was sorry I didn't get a trial done for her. It can depend on the BM's hair too, this BM had a lot of layers in hers which meant there was hardly anything to put up and she had not been growing them.