1. I would say this decision is a case-by-case basis. If you are changing a part that substantially affects the other drawings, I would say you should update the other drawings. If you are making a minor modification to a part that may show visually on the other drawings but doesn't substantially affect form-fit-function, then I would simply wait till the next revision on the affected drawings. Typically, others outside of engineering are not looking directly at your Creo models so they don't see the changes to sub-level parts (or at least that has been my experience).
2. #1 answers #2. If a change is warranted, a revision bump (or whatever your internal change management system requires) is necessary.
