Clicking on "Center Image" gets you close, but only centers the image on the printable area of the page. I, too, print on "letter" and simple have a print template that on a vertical, pushes the image down a tad and on a horizontal, pushes over from the left. Usually, I click in the box and hit the arrow key down or left two clicks and it's very close to the center of the paper.
An easier way may be this:
Make your canvas size the same as your paper size and when you tell it to print, toggle-off "Center Image" and set the top and left to "0". When it warns you that the print is too large for the paper, tell it ok.
With a template for printing that you paste your image into you'll save having to do this every time. Simply copy your pre-sized image and paste into the center of your output sheet template that you've saved with the print settings ready to go and you can print almost effortlessly.
