I don't know if it's generally considered a typesetting faux pas, but it personally doesn't bother me unless the lines above and below it are shorter than the line ending with "I," making the "I" float out in space by itself. In that case I would break it down to the next line, but I don't know if there's any hard-and-fast rule about it. You might want to experiment with your Paragraph Options - I think "Balance Ragged Lines" would get rid of that unsightly "I" in most cases.
I don't think it would bother me, either, but it would be pretty easy to set up a GREP style to find a capital I followed by a space and set them to a No Break style that would foce them onto the next line automatically.
I would most definitely be inclined to break it down to the next line, however you go about it, and all the more if you are using ragged right justification.
It's not quite so much of a sin as leaving a widow or an orphan, but it's usually unsightly and often unnecessary none the less.