I would say it depends on the series and episode. I am a lot more familiar with TOS and TNG, followed by (in order) DS9, Ent and Voyager. I do make corrections to clarify where I think something is lacking, or if there is something I find personally compelling that I want to emphasize. I do see that in other posts as well. Most of the time it's pretty straightforward CxV.
I think the compelling thing in a line by line is to appreciate the purity of the story as it exists as words on a page, you have to have good words to begin with. I enjoy reading and posting the line by line as it makes me appreciate the writers seminal contribution to the episode. Obviously some episodes (and series) are better than others in that regard.