I mostly leave images in the native 3:2 ratio for uploading to Flickr. If I'm printing then the ratio is the one needed to fit the paper/frame combination. I print up to A4 at home, and usually crop to leave a 5mm border all round. For some subjects printed in B&W though I will trim the A4 to 10×8 with a 0.2" border, just like I would have made the print in the darkroom. I get cheap frames with a 16×12 mat so use that rather than 18×12 which would also fit the frame, but the only alternative mat locally costs nearly what I pay for the glazed frame with mat. I do also do some images in other ratios, mostly square if I think it looks "right" or fits with the final use needs.