I guess what I do is work with the proportions of the film/camera I have with me to find scenes to get onto film. When using the Rolleicord, I am looking for square images. If I am carrying the 8x10, I am looking for images that fill the 8x10 proportions of the film holder -- but also for images that fill a 4x10 proportions since I also carry a modified darkslide that gives me two 4x10s on an 8x10 sheet of film.
So if I am walking around with the Rolleicord and see a "4x10" image, I wave at it, say, "see ya next time!" and move on and continue looking for square images. There are more out there than I have time to photograph and print, anyway.
If half of your subjects end up being printed square, then perhaps you should be using a square format. As to letting the subject dictate the ratio, you mean your eye dictates the ratio. However, if your eye is attuned to a specific format ratio, it may help to determine a given composition within that aspect ratio and improve your percentage of aesthetic capture.
Although I will crop an image however it needs to be done, I always try to compose for whatever format I am using. Working within a format's restrictions is part of the fun, like looking for patterns of color vs patterns of light and dark.