The answer is, it depends. It depends on the tolerance level in your workplace for embarrassing, misleading, confusing, and indecipherable written language. Proofreading is not copyediting. Proofreaders function within relatively strict limits. Proofreaders must know their authority level on each job.
Good proofreaders have certain qualities in common.
Methods Solo Team Machine-Assisted Procedure Understand key information. Mark the page correctly and consistently. Standard proofmarks Slashes Circles Printing or Writing Use a checklist if necessary. Be aware of things that complicate proofreading. Know when to correct and when to query.