I have enjoyed this thread very much, it includes a lot of good input.
Personally, the subject itself dictates to me how I want it to appear in the final finished print. I have long disliked the square format, but certainly wouldn't hesitate to use it if I felt that it would enhance or best display a
specific subject. My final choice of format based on the subject is made long before I make the initial exposure. My darkroom cropping is normally just inside the unexposed areas created by the film holder or camera back. I have never had anything to prove to anyone by making a print that includes the (to me) unsightly full frame sheet borders and code notches. That may be pure to some, but it's mud fence ugly to me.
A tough assignment to me is to be required to do a detailed photograph of a radio tower or other vertical subject in a horizontal format. I also find it very difficult to photograph an entire freight train using a vertical format. ;-)
My Idea is to choose what ever format is necessary to present the best possible finished image, regardless of the subject matter.