So, you just set out on a photo trek with your $3,000 camera set up, tripod, photographers vest, beret and clip on artists pony tail. You are the knee's of the bee as you visualize the shot before you. You have traveled far and at considerable expense to commit some serious art.
Now your only real concern is how to fit this not yet born photo into that 11x14 frame that you bought last week for a mere 99 cents at the yard sale.
Your dilemma is exactly that of DaVinci when Mr. Lisa hired him to paint his daughter Mona. Leo said to himself, "I know he has a favorite frame, so I must paint this thing to fit it".
STOP THINKING LIKE THAT.
Walmart and yard sales should not dictate your artistic choices. Shoot and crop to fit your vision. If that means you must spring some $ on custom framing, so be it.