I prefer to use every square cm of the neg, but certainly would not turn down an opportunity to create a new image from a neg by cropping...or to plan to crop to get the image I see with the the format I am using.

If one seems to be always doing some cropping to get a good image from a neg (and especially if the original compostion is not usable), then I would say that either the proportion of the format is not suitable for the way one sees, or one needs to concentrate on seeing more clearly (with one's format and lens/es) before clicking the shutter.

In my own work, I see the image before I set the camera up and refine it on the ground glass. This is usually not the case with the "average" 35mm camera user (especially with a zoom)...who composes primarily thru the viewfinder.