I have never heard that one before. When I was learning to type we always would margin to the left. Granted that was 45 years ago but it still holds true today in professional correspondence.
Jerry

As a graphic designer, I can tell you that justifying text is usually only good for corporate brochures or legal letters -- and even then it is questionable. Justified text does look very serious and off-putting, and yes, gives the illusion that there is more to read.
Flush left-text is much more user-friendly, especially for mobile devices, and is the way to go -- in my opinion.

Also, black text on white is much easier to read than white text on black. Not that we would do that here, but I've seen it done on other sites and it is very hard to read.

The use of fonts makes a difference as well. Fonts with serifs (Times Roman, Baskerville, Caslon) are usually used in more traditional settings, and give a more serious, traditional look, while fonts without serifs (Arial, Calibri, Open Sans, Helvetica) are for more up-dated and relevant designs.

Having said that, these rules obviously don't always apply -- you have to go with what you like.
/a.

you should never use justify on content which will be viewed on the phone. Format to the left and keep your images on the right