I generally repair grammar and punctuation mistakes if I notice them when re-reading. I also have occasionally made minor edits such as butane → propane when I realized that you can't really buy butane for a cooker in a walmart. However, sweeping changes are an absolute nono. For one thing I don't want to change the past too much because you never know where you're going to introduce a plot hole. Also I want my story to be finished at some point and constantly messing with linear time is not conducive to that. I have used the chapter reorganize feature exactly once to introduce new events between chapters in a way which could be interpreted as "yeah, there was just enough unaccounted time for that to have always had happened", and even that felt like cheating a bit.