Applying for posdtoc positions: is it okay to send out many applications?
You should remember that universities are also looking at the resumes of many candidates for a given position. If it is not rude on their part to consider many applications and rejecting most of them, why will it be rude for a candidate to apply to many positions and reject some of them?

This is a many-to-many matching market between applicants and positions. You are trying to optimize your chances, they are trying to optimize their interest. If you look at it from this game theoretic perspective, you will not feel any qualm of conscience.