What is a style guide, and why would you need one?
Well -- consider what happens when you need to use the following sorts of things more than once in your resume, cover letter and other documents:
- Did you earn a B.A. or a BA?
- Can you be reached at 716-555-1212, or at (716) 555-1212 or 716.555.1212?
- Will you be available for interviews after October 1, or after 01 October?
- Can you be reached at home after 5 p.m., or 5 PM, or 5 pm?
Accepted usage will change over time, and will vary from place to place. If you work for an organization that takes published written material seriously, you will be asked to follow a style guide--and the rules may change from one organization to the next.
When you're writing your resume and other application materials, you want to be certain that your usage is consistent. If telephone numbers on one page use hyphens, then telephone numbers on another page should not use parentheses around the area code. Anything else looks sloppy. I've supplied one link to one college's style guide online; you can see how many little picky choices there are to be made.
You don't have to use this or any other specific style guide. However, you can and should make conscious decisions about details. Nobody will notice if your formatting is consistent--but they will certainly notice if it is not.