Always.
Before publishing a text or handing it over to be evaluated, it must always be proofread. Publications, PhD theses and master’s degree final dissertations are usually proofread more thoroughly, as well as novels and literary works intended for publication, all kinds of translations into any language, letters in a language that the client does not know very well, etc.
It is advisable for another professional to proofread the text, since a second professional reader is more likely to detect any error the original author might have missed, whether it is an orthographic, terminological, punctuation or style mistake.