"Standard" image proportions are just someone else's idea of what works commercially. I find myself paying much attention to the edges of my images: what remains and what is left out. Those decisions do not always fit the 'standard' frame. Decades ago, I went through a phase when I had my enlarger masks machined so that I could print the edges of my negs, that was OK, but that limited me to 2:3; 4:5 and 1:1 proportions. It is worthwhile spending some time examining such things as the Fibonacci scale, or comparing picture or page proportions to musical intervals. Interestingly enough, the 2:3 proportion of the '35 mm full frame, 24 mm by 36 mm' works out to be the fifth musical interval: notes C - G. Spending time in art galleries and museums will also be worthwhile, as would picking upside textbooks on composition and such. Do not be hostage to pre-manufactured frames and mats.
When designing books, I refer very often to Robert Bringhurst's book 'The Elements of Typographic Style. Especially Chapter 8, Shaping the page.
Another idea is to make friends with a reputable framer and listen to her or his advice.
Jean-Michel