Chengyang is generally very safe in any time. Drink only bottled water and wash carefully everything you intend to eat fresh, and that's about it.
fees/permits
The entrance fee is ¥60, student card can get a discount price of ¥30. Note that until recently the local villagers hadn't seen a penny from this money, which, officially "used for maintaining the bridge", in fact went to the pockets of the local officials. After endless petitions from the locals, the authorities guaranteed that they will get 30% of the income gained buy the entrance fees collection. In fact, no-one checks the books, so the actual part might easily be 10%, or even 1%. Therefore, if you manage to avoid paying the entrance fee by continuing with a bus just 100 meters further on from the main entrance and entering the area "from the back door", the only ones you'll be cheating are the corrupted communists, and this is a blessing.
The fee to get into the village is ¥60. However, if the bus from Sanjiang travels about a kilometer further, one can enter the village from the highway without paying the fee. Tell the bus driver to drive you past. Also, if you arrive in the village before 0730 in the morning, the guards will not be there, and one can enter free. However, the "policeman" do patrol the small community and may require the fee.
If you ask for Chengyang on the bus, or get a taxi, the driver will drop you at the ticket office to pay. Hostel managers also have to take guests passports to register with the police. Dodging the entrance fee, it seems, is becoming increasingly difficult.