I am a believer in presenting art in the size and aspect ratio that best compliments it. I know that we are a world of standards, and there are some very common and readily available aspect ratios such as 3:2, 4:3, 4:5, 1:1. While a standard set of well-established formats makes it easy to produce printers and papers that meet the average persons needs in a cost effective way...I don't really think it best services the needs of art itself.
If you have some great photos in an odd-ball format, I so go with it. Have a 1:8 size vertical image? Print it in a 1:8 format, and frame it as such. Have some complimentary 3:1 panoramic shots? Print em in 3:1 canvas, and display em in a vertical triptych format. Or, for that matter, split a single 3:1 panoramic into a 3 slice horizontal triptych.
If some prints look better huge, and others look better small, then display them that way. You can put your complimentary images together in a random collage, or you could even print complimentary images such that a variety of formats fit together into a cohesive whole. An example of this could be a 3:1 panoramic, a couple 2:1 rectangulars, and a couple 1:1 squares put together in a square composition.
I guess my approach to displaying art is that the display is a form of art itself. There shouldn't be any specific rules that dictate what is or isn't appropriate for every photo you display. Every photo should, and probably will, dictate its own best form of presentation. Sometimes you need to experiment with a variety of framing and mounting options to find the form that best compliments each specific piece. Sometimes that means putting a photo in a nice frame with complimentary matting. Sometimes it means a simple frame with standard white matting. Sometimes it means canvas or triptych, and others it may mean art block mounting by laminating it strait to a piece of MDF.
It is probably also important not to overlook the effect of the display environment on the form of display. Some forms of mounting may not work well with a particular environment, such as a gallery. Some galleries may work better with consistent mounting, while others may lend themselves better to a variety of display forms.
