I do the same, except in circumstances when I know I will crop a certain way when I print the picture. In these cases, I compose precisely in one dimension, but not the other.
I tend to leave a little bit of a gutter with sheet film, if possible. Perhaps 1/8 inch. You don't lose much quality by cropping these formats, and the damage-and-light-leak-prone edges are less likely to end up in the print this way. Also, the condenser of my 4x5 enlarger barely covers an entire sheet of 4x5, and my 4x5 negative carrier doesn't allow me to print quite all of the full frame anyhow.
I don't have any qualms about cropping a picture that was not as well composed as I would have liked. This is usually with unrepeatable grab shots, of course, when other considerations were more important than exact composition when the picture was taken.