I came across this problem the other day when using WordPress auto embed feature. When you add an embed URL, for example “http://youtu.be/M-aGPniFvS0 ..” it will automatically be aligned to the left (this depends on the theme). This can look odd depending on your layout, there is a trick to make it centered in the post by doing the following in the text editor:
<div style="text-align: center;"> ... embed url ... </div>
It’s important that the div and the embed URL are all on separate lines.
Beware: Also there is a major drawback in that if you update the post again it will auto format the markup and remove the line spaces and the auto embed will not work.