I think it depends on what you’re fixing. A good general rule is to leave a regular comment (in addition to an edit summary) if you’re changing something important or a lot of things.
Editing posts that are going to be deleted is often a waste of time (never edit spam posts).
Editors can edit the tags as much as is necessary, even if that means changing them completely. Tags are hard and there’s almost always room for improvement if you know what you’re doing. This is about helping everyone find things since it’s already pretty hard to do plenty of searches given the subject matter.
I try to fix mistakes in titles (except if it’s part of what’s being asked about), since it looks good if the question shows up in the HNQ or somewhere else.
However, most titles are terrible, to the point where I’m often rewriting titles completely. It’s not really about English proficiency either. Titles like “I have a question about the meaning of this verb”, while grammatically correct could be applied to many different questions and should be replaced with something that both describes the question and (to the fullest extent possible) uniquely identifies it. For example a good title would be “What does ‘have at it’ mean?” I try to keep the the title simple and not use words OP wouldn’t know (which is important for word requests).
Titles are very important. It’s weighed more than the body for searches and it’s also what is used to find potentially related questions when asking questions.
For answers, fix all the grammar and spelling. If the answer is factually wrong, don’t fix it. Comment instead.
For questions I avoid fixing grammar here so that answerers have a good idea of the level of the OP’s English. But there is still quite a bit that I would fix:
- Formatting. This covers a lot of different stuff, such as adding line breaks, removing inappropriate code format and changing bare urls to
[text](url). Another pretty important thing this includes is the transcription of images and adding of sources (and sometimes more context) for quotes. None of this really indicates the proficiency of the user in English, only in their ability to understand the software.
- Spelling, except where it’s the focus of the question. SE search is very sensitive to spelling so mistakes like “propositional phrases” only hurt in the long run. If it looks like the OP doesn’t know the correct spelling then I leave a comment explaining why I fixed things.
- Digressions. If the question is being bogged down by a lot of information that is not relevant to the question, then it’s often wise to remove it. This also includes removing unnecessary parts of overly long quotes. (ELL questions are often short enough that this isn’t a problem.)
I’m not sure how well this aligns with what everyone else thinks, so I’ll be curious to hear any feedback. I don’t see it being different than the consensus, except that it’s more detailed.