Gregory's Server@xenophora I think it's much more acceptable when it's made clear that I'm not captioning "for" them, I'm adding my description of the image as I understand it. It's why I might reply with a caption without @-ing the op, or write a separate caption post linking the boosted post.
That said, I don't bother as much these days and simply don't interact with undescribed media for the most part. Most media on my TL is described, and there's plenty to boost without going out of my way adding a description no one asked for.
@ljwrites @skye
Sounds good to me.
I'll occasionally boost something without a description, if I think it's important for some other reason. But usually I just settle for putting a *star* on it. It's possible the person has their own reasons for not describing and I'd rather not be Board Cop.