Applying Rust Killer to under carriage /frame any reason not to ?

I agree, I'm thinking it depends on how long the piece sits in the acidic solution for? An extended period may need more neutralizing?
I understand pH and how neutralize an acid. I was just wondering if people actually rinse their parts with a basic solution after soaking in phosphoric acid. To me, that'd be a total waste of time. A simple water rinse works fine.