Whatever you want.. but I personally find it easier to stick with a few standard-to-me ratios (3:2, 4:3, 16:9, 255:100, 2.125:3.5). I get around printing non-standard ratios by having appropriate export presets in Lightroom that add the required border in black, white or mid-grey.
I wouldn't change a photo to fit the paper. Paper can be cut as easily as a photo can be cropped.