referencing this issue and this one you can use doctrine's @OrderBy annotation to have the Tags ordered when fetched from database. This will affect the form-collection rendering aswell.
/**
* @ORM\ManyToMany(targetEntity="Tag")
* @ORM\OrderBy({"position" = "ASC"})
*/
protected $tags;
Just as a quick reminder you can order already fetched collections in your controller as described in my answer here.