I'm going to offer a CS perspective from Germany, which may apply to other engineering fields, as well:
While there is quite some writing in CS, within my circle of academic contacts, we usually go by the rule: A few minor mistakes are sloppy and irritating, but are to be expected in long documents and therefore excusable. What is not excusable is failing to have someone else proofread the document. This is particularly (but not exclusively) true when writing in a foreign language, such as (for us) English.
So, in other words, having someone else proofread each of your more significant documents (theses, seminar papers, ...) is considered a part of due diligence. This is, of course, not limited to student assignments, but will continue during one's professional life - authors of a paper should always give the camera-ready version of a research paper to someone so far uninvolved to do some proofreading, for instance.
In the student context, preferrably, this proofreading should be done by a fellow student:
- They are likely to be savvy about the topic. This reduces the chance of false positives, where they consider something incorrectly written or expressed, which is actually correct in the terminology of the subject at hand.
- It is a good practice for them; like that, they will not just train writing, but also proofreading/correcting, which they will have to do efficiently also later on.
- As all students have to do the assignments, and all students need proofreaders, this allows for inherent compensation to some extent, as students can simply thank each other by returning the favour of proofreading.
As other answers remarked, it is important that proofreading is really that - identify mistakes, point out unclear or confusing statements and document structure, possibly suggest how to improve the text on an abstract level, but do not change anything other than the simplest of mistakes (straightforward typos that leave only one option for correction).
Concerning the attribution, I am undecided on whether it is obligatory:
- It is certainly the polite thing to do.
- In longer documents that feature acknowledgments (such as graduation theses), those acknowledgments provide a good opportunity for mentioning that the author received help by proofreaders. (Of course, not everyone puts acknowledgments in graduation theses; especially when the deadline is closing in, there are more essential parts of the document to work on.)
- Given that proofreading is, as explained above, an indispensable part of writing a document, and a rather "technical" one (just like, e.g., using a text editor), it is debatable whether it should be mentioned. Maybe it should, but in that case, the producers of one's text editing software, or the manufacturer of one's computer, should probably be mentioned just as well, which is rarely done.
- In shorter documents (especially papers with a page restriction), there is most probably simply no space to mention mere operational details of the paper writing process such as proofreading.
To summarize, I have rarely seen mentions of proofreading in documents, even though they occasionally do occur. Especially in student documents, when I do not find any mention of proofreading and I ask the student whether they have had someone else proofread their document, I am definitely much more dissatisfied with the answer "No, that's why I didn't mention it." than with the answer "Yes, of course; having someone else proofread my document before I hand it in is so self-evident that I do not see a point in specifically mentioning it."
EDIT: Whether or not to add an attribution may also depend on cultural customs. I recently brought up the topic concerning one of our papers in my department in Germany, when we had handed out that paper to a few colleagues from other departments for checking comprehensibility and clarity of the text (i.e. not restricted to spellchecking, but actual contents of the paper). My tentative suggestion to add a note in the acknowledgments thanking them for their suggestions was rejected as "silly" by my co-authors, because the colleagues from the other department "have just read the paper and suggested a few minor changes", and because "everyone always gives other people one's papers for external suggestions and those external people are normally never mentioned". People from other cultures might agree on different courses of action that are "normally" followed.