Avoid grep functions.
This actually won't work. The docstring specifically says "commands", not functions. I have another branch where I'm testing a custom variable that you can add any function to: https://github.com/DarwinAwardWinner/ido-ubiquitous/commits/func-except. It seems to be working, and I'll be submitting a pull request soon after some more testing.
In the meantime, the proper solution is (disable-ido-ubiquitous-in grep-read-files).
Hm; I tested it before committing and it seemed to work fine, but then I realized I hadn't tested it without this change. Turns out it's unnecessary; rgrep works fine already. Is it the read-file-name disable call at the bottom that's causing this?