Current nitrile O-rings are perfectly safe with lithium grease.
Lithium grease is only applied to stabilizer parts and will be never come in contact with the O-ring. However, precaution should be used when handling the O-rings and grease. I recommend that you wash your hands before handling the EPDM O-rings. EPDM will swell if you get grease on it. I don't have them on hand yet, but I will do some testing to see how sensitive they really before putting them on the website.
I don't know what these cheap O-rings from the hardware store are, but I've looked at several O-ring vendors and quality O-rings are not cheap even when I purchase 20,000-40,000 at a time.