According to Robin Williams' “The Non-Designer’s Design Book,” the principle of proximity states that you group related items together. When creating a flyer, a brochure, or a newsletter, try to form a list into visual groups. Add some contrast to the headlines, use extra margins, underline the relationships, and emphasize important information.
The next principle of design is alignment. "The principle of alignment states that nothing should be placed on the page arbitrarily. Every item should have a visual connection with something else on the page... Even when aligned elements are physically separated from each other, there is an invisible line that connects them, both in your eye and in your mind."
My students often use a centered alignment for the posters, and it works great.
However, when you look at 15-20 different posters with the centered alignment, it gets boring. According to Williams, “a centered alignment creates a more formal look, a more sedate look, a more ordinary and oftentimes downright dull look.” She suggests using “a strong flush right or left alignment with good use of proximity.”
This business card has a centered alignment.
The strong flush-right gives the card the "hard" edge. "The strength of this edge is what gives strength to the layout."
My students use Illustrator guides to align objects.
Illustrator has also the Align window.
An alignment choice for posters often depends on the visual elements. Henri de Toulouse-Lautrec, a true genius of the posters, used what I would call a creative alignment.
I agree with Williams “that most designs that have a sophisticated look are not centered.” Based on my observation, the students’ projects with the creative alignment have a refined look.