Which is best for headshots? Horizontal or vertical? The answer is...it doesn’t matter.
Design trends are changing, online profiles are changing, social media has certain ratio requirements for thumbnails and photos, theatre will always want a hard copy as will many casting directors, still. If you're printing, you must still use 8x10 paper, have your name on or below the photo and, I probably don't need to say this, but be sure to print in color. Other than that, the standards and trends are fluid. The strict rules that were once considered headshot gospel no longer apply.
Whenever I'm in a session, I make sure that we shoot for as many crop ratios as we can muster within each look. It's important to me that each client leave the session with more than enough marketing tools to populate all of their online profiles, social media pages, as well as their hard copy needs.
Here's a handy dandy cheat sheet if you're trying to figure out the ideal ratios for all of your social media pages: http://bit.ly/2eJqDJB
Next up? Studio light or Natural light. Which is best? (I'll bet you already know the answer.)