When somebody posts a video to YouTube, and they do their settings, there's an option to allow others to re-post the videos (repost them anywhere I think.) If they have not chosen the option to allow others to use their videos, you won't be able to get the embed code - so, generally, if you are able to get the embed code, they've allowed you to re-post their video. However, I'm not sure what the default setting is when people upload videos to YouTube - if the default setting is that people can access the embed code, and the person who uploaded the video didn't actually want anybody else using their videos elsewhere, that seems a little "iffy." Although, if people are using YouTube to upload their videos in the first place, they should be aware of all the rules, and should also know how to make use of all the settings. I'm not sure what would happen if somebody moaned at you for using their YouTube video - would telling them that they didn't block the video's embed code be enough to calm them down?