ok, but do you try to stick to standard sizes?
OK, you got me there.
I admit that I am somewhat inconsistent about cropping, and I do let practical considerations get in the way some of the time. For example:
1. My initial (in-camera) cropping is often dictated at least partly by the shape of the viewfinder/sensor/film. When using a view camera (4x5" or 8x10"), I tend to look for scenes that fit well into the 4x5 format. My 35mm images are often wider, having about a 2x3 format (from the 24x36 mm negative format.) Much to my surprise, when I began using a 6x6 cm medium format camera, I found lots of images that seemed to want to be square.
2. But, I often find images that do not
fit nicely into the camera's native format. I will take the picture anyway, with the conscoious intention of cropping into a non-standard size later. The first print I make of any image is most often cropped in a way that seems to fit the image, regardless of how conventional the proportions are.
3. Another problem comes up when I am preparing pictures for an exhibition. To my eyes it looks kind of bad to have just one or two images in unusual proportions. And I have standard sized frames and overmats that I like to reuse from one show to the next (saves $$$), so I may try to select images that all seem to work in a common size. Lately I have been showing many prints cropped to 10x15" (in 16x20" frames) or 12x18" (in 18x23" frames). Some of my photos are sufficiently abstract that I can get away with stretching the images in one direction to make them fit one of my standard sizes. I won't do that with landscapes or images of people, as that can get really creepy really fast.
This is probably more detail than you really wanted, but I guess the bottom line is: like the "rule of thirds" for composition, I think you should feel free to use a standard size (or the golden ratio) if it really seems to fit your image; otherwise, use the cropping that fits the subject.
My 2 cents, YMMV.