Either 180天 or 183天 is OK. Nothing affected.

怎么机票代理说往返机票/不能超过180天呢?
-I think it is because B2 holder normally is only allowed to stay in US for 6 months unless he extends the B2 status.

签证有效期6个月,入境给半年,如何买机票往返日期?
-签证有效期签证有效期6个月 means your parents need to enter US within 6 months with the visa, and it has nothing to do with how long they will stay in US.

入境给半年, 能用父母探亲报税?
-I do not know.