There are 2 parts to the question and answer really.
A parish councillor does not have to declare, on the planning application form that they are a PARISH councillor since it is the planning authority not the PC which determines planning applications. So the first half of the answer is no, a parish councillor would not need to declare that they are a parish councillor on a planning application. If they were a Cllr or employee of the higher tier council which delivers the planning authority function then they would.
Part 2 although not having to declare them self as a Cllr or employee (unless they are) on the application to the planning authority, they would have to declare an interest if said planning applicants were considered by the parish council upon which they sit.