read my posts in the follow thread..
Develop photos with white border
Adding border on photos to send for printing
Border for photos...
the correct method is get the image into paper size with correct ratio first,
make the image in a separate layer, the bottom layer in white or whatever color you want
scale down the image layer without changing the canvas size to add borders
the image should reveal the color of the bottom layer
but the four borders are in different thickness now
Flatten, Ctrl A and use stroke command (inside) to add just enough border for making four borders in same thickness.
voila!
for this method you can just create just ONE action for images in portrait or landscape orientation.
However, you may still get uneven borders even your images are done perfectly, it can because of the lab does not make the paper tray sit properly, or whatever other reasons.