Interesting. Thanks for your reply.
The pinhole camera I use has a very good pinhole. It's sharp and clean, and I'm amazed at how nice the prints look when I make the most use of the contrast available in the paper. Even more so than with lens photography, I find that getting the negative 'just right' is even more important, because I feel like I rely on tonal values for sharpness.
I haven't had to clean my pinhole yet, after almost two years of using it. Perhaps it's time to check it with a microscope!
Thanks again for your advice.