The Common category includes the following blocks: Paragraph, image, headings, list, gallery, quote, audio, cover, video. The paragraph block is the default block type.  It should not have any alignment of any kind. It should just flow like you would normally expect. Nothing fancy.

This post tests various embed blocks:

Really cool to read through and find so much awesomeness added to WordPress 3.6 while I was gone. I should take three weeks off more often. — Andrew Nacin (@nacin) April 3, 2013 This post tests WordPress’ Twitter Embeds feature.